The Botanic Garden
by
Erasmus Darwin
Part 6 out of 7
quantity of heterogeneous substances fill up the chasm which disjoins
them, on one side of this chasm the seven strata of coal are seen
corresponding in respect to their reciprocal thickness and goodness with
the seven strata on the other side of the cavity, except that they have
been elevated several yards higher. Phil. Trans. No. 360. abridg. Vol.
V. p. 237.
The cracks in the coal-bed near Ticknall in Derbyshire, and in the sand-
stone rock over it, in both of which specimens of lead-ore and spar are
found, confirm this opinion of their having been forcibly raised up by
subterraneous fires. Over the colliery at Brown-hills near Lichfield,
there is a stratum of gravel on the surface of the ground; which may be
adduced as another proof to shew that those coals had some time been
beneath the sea, or the bed of a river. Nevertheless, these arguments
only apply to the collieries above mentioned, which are few compared
with those which bear no marks of having been immersed in the sea.
On the other hand the production of coals from morasses, as described in
note XX. is evinced from the vegetable matters frequently found in them,
and in the strata over them; as fern-leaves in nodules of iron-ore, and
from the bog-shells or fresh water muscles sometimes found over them, of
both which I have what I believe to be specimens; and is further proved
from some parts of these beds being only in part transformed to coal;
and the other part still retaining not only the form, but some of the
properties of wood; specimens of which are not unfrequent in the
cabinets of the curious, procured from Loch Neigh in Ireland, from Bovey
near Exeter, and other places; and from a famous cavern called the
Temple of the Devil, near the town of Altorf in Franconia, at the foot
of a mountain covered with pine and savine, in which are found large
coals resembling trees of ebony; which are so far mineralized as to be
heavy and compact; and so to effloresce with pyrites in some parts as to
crumble to pieces; yet from other parts white ashes are produced on
calcination, from which _fixed alcali_ is procured; which evinces their
vegetable origin. (Dict. Raisonné, art. Charbon.) To these may be added
another argument from the oil which is distilled from coals, and which
is analogous to vegetable oil, and does not exist in any bodies truly
mineral. Keir's Chemical Dictionary, art. Bitumen.
Whence it would appear, that though most collieries with their attendant
strata of clay, sand-stone, and iron, were formed on the places where
the vegetables grew, from which they had their origin; yet that other
collections of vegetable matter were washed down from eminences by
currents of waters into the beds of rivers, or the neighbouring seas,
and were there accumulated at different periods of time, and underwent a
great degree of heat from their fermentation, in the same manner as
those beds of morass which had continued on the plains where they were
produced. And that by this fermentation many of them had been raised
from the ocean with sand and sea-shells over them; and others from the
beds of rivers with accumulations of gravel upon them.
4. For the purpose of bringing this history of the products of morasses
more distinctly to the eye of the reader, I shall here subjoin two or
three accounts of sinking or boring for coals, out of above twenty which
I have procured from various places, though the terms are not very
intelligible, being the language of the overseers of coal-works.
1. _Whitfield mine_ near the Pottery in Staffordshire. Soil 1 foot.
brick-clay 3 feet. shale 4. metal which is hard brown and falls in the
weather 42. coal 3. warrant clay 6. brown gritstone 36. coal 31/2. warrant
clay 31/2. bass and metal 531/2. hardstone 4. shaly bass 11/2. coal 4.
warrant clay, depth unknown. in all about 55 yards.
2. _Coal-mine at Alfreton_ in Derbyshire. Soil and clay 7 feet.
fragments of stone 9. bind 13. stone 6. bind 34. stone 5. bind 2. stone
2. bind 10. coal 11/2. bind 11/2. stone 37. bind 7. soft coal 3. bind 3.
stone 20. bind 16. coal 71/2. in all about 61 yards.
3. _A basset coal-mine at Woolarton_ in Nottinghamshire. Sand and gravel
6 feet. bind 21. stone 10. smut or effete coal 1. clunch 4. bind 21.
stone 18. bind 18. stone-bind 15. soft coal 2. clunch and bind 21. coal
7. in all about 48 yards.
4. _Coal-mine at West-Hallam_ in Nottinghamshire. Soil and clay 7 feet.
bind 48. smut 11/2. clunch 4. bind 3. stone 2. bind 1. stone 1. bind 3.
stone 1. bind 16. shale 2. bind 12. shale 3. clunch, stone, and a bed of
cank 54. soft coal 4. clay and dun 1. soft coal 41/2. clunch and bind 21.
coal 1. broad bind 26. hard coal 6. in all about 74 yards.
As these strata generally lie inclined, I suppose parallel with the
limestone on which they rest, the upper edges of them all come out to
day, which is termed bassetting; when the whole mass was ignited by its
fermentation, it is probable that the inflammable part of some strata
might thus more easily escape than of others in the form of vapour; as
dews are known to slide between such strata in the production of
springs; which accounts for some coal-beds being so much worse than
others. See note XX.
From this account of the production of coals from morasses it would
appear, that coal-beds are not to be expected beneath masses of lime-
stone. Nevertheless I have been lately informed by my friend Mr. Michell
of Thornhill, who I hope will soon favour the public with his geological
investigations, that the beds of chalk are the uppermost of all the
limestones; and that they rest on the granulated limestone, called
ketton-stone; which I suppose is similar to that which covers the whole
country from Leadenham to Sleaford, and from Sleaford to Lincoln; and
that, thirdly, coal-delphs are frequently found beneath these two
uppermost beds of limestone.
Now as the beds of chalk and of granulated limestone may have been
formed by alluviation, on or beneath the shores of the sea, or in
vallies of the land; it would seem, that some coal countries, which in
the great commotions of the earth had been sunk beneath the water, were
thus covered with alluvial limestone, as well as others with alluvial
basaltes, or common gravel-beds. Very extensive plains which now consist
of alluvial materials, were in the early times covered with water; which
has since diminished as the solid parts of the earth have increased. For
the solid parts of the earth consisting chiefly of animal and vegetable
recrements must have originally been formed or produced from the water
by animal and vegetable processes; and as the solid parts of the earth
may be supposed to be thrice as heavy as water, it follows that thrice
the quantity of water must have vanished compared with the quantity of
earth thus produced. This may account for many immense beds of alluvial
materials, as gravel, rounded sand granulated limestone, and chalk,
covering such extensive plains as Lincoln-heath, having become dry
without the supposition of their having been again elevated from the
ocean. At the same time we acquire the knowledge of one of the uses or
final causes of the organized world, not indeed very flattering to our
vanity, that it converts water into earth, forming islands and
continents by its recrements or exuviae.
NOTE XXIV.--GRANITE.
_Climb the rude steeps, the Granite-cliffs surround._
CANTO II. l. 523.
The lowest stratum of the earth which human labour has arrived to, is
granite; and of this likewise consists the highest mountains of the
world. It is known under variety of names according to some difference
in its appearance or composition, but is now generally considered by
philosophers as a species of lava; if it contains quartz, feltspat, and
mica in distinct crystals, it is called granite; which is found in
Cornwall in rocks; and in loose stones in the gravel near Drayton in
Shropshire, in the road towards Newcastle. If these parts of the
composition be less distinct, or if only two of them be visible to the
eye, it is termed porphyry, trap, whinstone, moorstone, slate. And if it
appears in a regular angular form, it is called basaltes. The affinity
of these bodies has lately been further well established by Dr. Beddoes
in the Phil. Trans. Vol. LXXX.
These are all esteemed to have been volcanic productions that have
undergone different degrees of heat; it is well known that in Papin's
digester water may be made red hot by confinement, and will then
dissolve many bodies which otherwise are little or not at all acted upon
by it. From hence it may be conceived, that under immense pressure of
superincumbent materials, and by great heat, these masses of lava may
have undergone a kind of aqueous solution, without any tendency to
vitrification, and might thence have a power of crystallization, whence
all the varieties above mentioned from the different proportion of the
materials, or the different degrees of heat they may have undergone in
this aqueous solution. And that the uniformity of the mixture of the
original earths, as of lime, argil, silex, magnesia, and barytes, which
they contain, was owing to their boiling together a longer or shorter
time before their elevation into mountains. See note XIX. art. 8.
The seat of volcanos seems to be principally, if not entirely, in these
strata of granite; as many of them are situated on granite mountains,
and throw up from time to time sheets of lava which run down over the
proceeding strata from the same origin; and in this they seem to differ
from the heat which has separated the clay, coal, and sand in morasses,
which would appear to have risen from a kind of fermentation, and thus
to have pervaded the whole mass without any expuition of lava.
[Illustration: _Section of the Earth. A sketch of a supposed Section of
the Earth in respect to the disposition of the Strata over each other
without regard to their proportions or number. London Published Dec'r
1st 1791 by J. Johnson St Paul's Church Yard._]
All the lavas from Vesuvius contain one fourth part of iron, (Kirwan's
Min.) and all the five primitive earths, viz. calcareous, argillaceous,
siliceous, barytic, and magnesian earths, which are also evidently
produced now daily from the recrements of animal and vegetable bodies.
What is to be thence concluded? Has the granite stratum in very antient
times been produced like the present calcareous and siliceous masses,
according to the ingenious theory of Dr. Hutton, who says new continents
are now forming at the bottom of the sea to rise in their turn, and that
thus the terraqueous globe has been, and will be, eternal? Or shall we
suppose that this internal heated mass of granite, which forms the
nucleus of the earth, was a part of the body of the sun before it was
separated by an explosion? Or was the sun originally a planet, inhabited
like ours, and a satellite to some other greater sun, which has long
been extinguished by diffusion of its light, and around which the
present sun continues to revolve, according to a conjecture of the
celebrated Mr. Herschell, and which conveys to the mind a most sublime
idea of the progressive and increasing excellence of the works of the
Creator of all things?
For the more easy comprehension of the facts and conjectures concerning
the situation and production of the various strata of the earth, I shall
here subjoin a supposed section of the globe, but without any attempt to
give the proportions of the parts, or the number of them, but only their
respective situation over each other, and a geological recapitulation.
GEOLOGICAL RECAPITULATION.
1. The earth was projected along with the other primary planets from the
sun, which is supposed to be on fire only on its surface, emitting light
without much internal heat like a ball of burning camphor.
2. The rotation of the earth round its axis was occasioned by its
greater friction or adhesion to one side of the cavity from which it was
ejected; and from this rotation it acquired its spheroidical form. As it
cooled in its ascent from the sun its nucleus became harder; and its
attendant vapours were condensed, forming the ocean.
3. The masses or mountains of granite, porphery, basalt, and stones of
similar structure, were a part of the original nucleus of the earth; or
consist of volcanic productions since formed.
4. On this nucleus of granite and basaltes, thus covered by the ocean,
were formed the calcareous beds of limestone, marble, chalk, spar, from
the exuviae of marine animals; with the flints, or chertz, which
accompany them. And were stratified by their having been formed at
different and very distant periods of time.
5. The whole terraqueous globe was burst by central fires; islands and
continents were raised, consisting of granite or lava in some parts, and
of limestone in others; and great vallies were sunk, into which the
ocean retired.
6. During these central earthquakes the moon was ejected from the earth,
causing new tides; and the earth's axis suffered some change in its
inclination, and its rotatory motion was retarded.
7. On some parts of these islands and continents of granite or limestone
were gradually produced extensive morasses from the recrements of
vegetables and of land animals; and from these morasses, heated by
fermentation, were produced clay, marle, sandstone, coal, iron, (with
the bases of variety of acids;) all which were stratified by their
having been formed at different, and very distant periods of time.
8. In the elevation of the mountains very numerous and deep fissures
necessarily were produced. In these fissures many of the metals are
formed partly from descending materials, and partly from ascending ones
raised in vapour by subterraneous fires. In the fissures of granite or
porphery quartz is formed; in the fissures of limestone calcareous spar
is produced.
9. During these first great volcanic fires it is probable the atmosphere
was either produced, or much increased; a process which is perhaps now
going on in the moon; Mr. Herschell having discovered a volcanic crater
three miles broad burning on her disk.
10. The summits of the new mountains were cracked into innumerable
lozenges by the cold dews or snows falling upon them when red hot. From
these summits, which were then twice as high as at present, cubes and
lozenges of granite, and basalt, and quartz in some countries, and of
marble and flints in others, descended gradually into the valleys, and
were rolled together in the beds of rivers, (which were then so large as
to occupy the whole valleys, which they now only intersect;) and
produced the great beds of gravel, of which many valleys consist.
11. In several parts of the earth's surface subsequent earthquakes, from
the fermentation of morasses, have at different periods of time deranged
the position of the matters above described. Hence the gravel, which was
before in the beds of rivers, has in some places been raised into
mountains, along with clay and coal strata which were formed from
morasses and washed down from eminences into the beds of rivers or the
neighbouring seas, and in part raised again with gravel or marine shells
over them; but this has only obtained in few places compared with the
general distribution of such materials. Hence there seem to have existed
two sources of earthquakes, which have occurred at great distance of
time from each other; one from the granite beds in the central parts of
the earth, and the other from the morasses on its surface. All the
subsequent earthquakes and volcanos of modern days compared with these
are of small extent and insignificant effect.
12. Besides the argillaceous sand-stone produced from morasses, which is
stratified with clay, and coal, and iron, other great beds of siliceous
sand have been formed in the sea by the combination of an unknown acid
from morasses, and the calcareous matters of the ocean.
13. The warm waters which are found in many countries, are owing to
steam arising from great depths through the fissures of limestone or
lava, elevated by subterranean fires, and condensed between the strata
of the hills over them; and not from any decomposition of pyrites or
manganese near the surface of the earth.
14. The columns of basaltes have been raised by the congelation or
expansion of granite beds in the act of cooling from their semi-vitreous
fusion.
NOTE XXV.--EVAPORATION.
_Aquatic nymphs! you lead with viewless march
The winged vapour up the aerial arch._
CANTO III. l. 13.
I. The atmosphere will dissolve a certain quantity of moisture as a
chemical menstruum, even when it is much below the freezing point, as
appears from the diminution of ice suspended in frosty air, but a much
greater quantity of water is evaporated and suspended in the air by
means of heat, which is perhaps the universal cause of fluidity, for
water is known to boil with less heat in vacuo, which is a proof that it
will evaporate faster in vacuo, and that the air therefore rather
hinders than promotes its evaporation in higher degrees of heat. The
quick evaporation occasioned in vacuo by a small degree of heat is
agreeably seen in what is termed a pulse-glass, which consists of an
exhausted tube of glass with a bulb at each end of it and with about two
thirds of the cavity filled with alcohol, in which the spirit is
instantly seen to boil by the heat of the finger-end applied on a bubble
of steam in the lower bulb, and is condensed again in the upper bulb by
the least conceivable comparative coldness.
2. Another circumstance evincing that heat is the principal cause of
evaporation is that at the time of water being converted into steam, a
great quantity of heat is taken away from the neighbouring bodies. If a
thermometer be repeatedly dipped in ether, or in rectified spirit of
wine, and exposed to a blast of air, to expedite the evaporation by
perpetually removing the saturated air from it, the thermometer will
presently sink below freezing. This warmth, taken from the ambient
bodies at the time of evaporation by the steam, is again given out when
the steam is condensed into water. Hence the water in a worm-tub during
distillation so soon becomes hot; and hence the warmth accompanying the
descent of rain in cold weather.
3. The third circumstance, shewing that heat is the principal cause of
evaporation, is, that some of the steam becomes again condensed when any
part of the heat is withdrawn. Thus when warmer south-west winds replete
with moisture succeed the colder north-east winds all bodies that are
dense and substantial, as stone walls, brick floors, &c. absorb some of
the heat from the passing air, and its moisture becomes precipitated on
them, while the north-east winds become warmer on their arrival in this
latitude, and are thence disposed to take up more moisture, and are
termed drying winds.
4. Heat seems to be the principal cause of the solution of many other
bodies, as common salt, or blue vitriol dissolved in water, which when
exposed to severe cold are precipitated, or carried, to the part of the
water last frozen; this I observed in a phial filled with a solution of
blue vitriol which was frozen; the phial was burst, the ice thawed, and
a blue column of cupreous vitriol was left standing upright on the
bottom of the broken glass, as described in note XIX.
II. Hence water may either be dissolved in air, and may then be called
an aerial solution of water; or it may be dissolved in the fluid matter
of heat, according to the theory of M. Lavoisier, and may then be called
steam. In the former case it is probable there are many other vapours
which may precipitate it, as marine acid gas, or fluor acid gas. So
alcaline gas and acid gas dissolved in air precipitate each other,
nitrous gas precipitates vital air from its azote, and inflammable gas
mixed with vital air ignited by an electric spark either produces or
precipitates the water in both of them. Are there any subtle exhalations
occasionally diffused in the atmosphere which may thus cause rain?
1. But as water is perhaps many hundred times more soluble in the fluid
matter of heat than in air, I suppose the eduction of this heat, by
whatever means it is occasioned, is the principal cause of devaporation.
Thus if a region of air is brought from a warmer climate, as the S.W.
winds, it becomes cooled by its contact with the earth in this latitude,
and parts with so much of its moisture as was dissolved in the quantity
of calorique, or heat, which it now looses, but retains that part which
was suspended by its attraction to the particles of air, or by aerial
solution, even in the most severe frosts.
2. A second immediate cause of rain is a stream of N.E. wind descending
from a superior current of air, and mixing with the warmer S.W. wind
below; or the reverse of this, viz. a superior current of S.W. wind
mixing with an inferior one of N.E. wind; in both these cases the whole
heaven becomes instantly clouded, and the moisture contained in the S.W.
current is precipitated. This cause of devaporation has been ingeniously
explained by Dr. Hutton in the Transact. of Edinburgh, Vol. I, and seems
to arise from this circumstance; the particles of air of the N.E. wind
educe part of the heat from the S.W. wind, and therefore the water which
was dissolved by that quantity of _heat_ is precipitated; all the other
part of the water, which was suspended by its attraction to the
particles of air, or dissolved in the remainder of the heat, continues
unprecipitated.
3. A third method by which a region of air becomes cooled, and in
consequence deposits much of its moisture, is from the mechanical
expansion of air, when part of the pressure is taken off. In this case
the expanded air becomes capable of receiving or attracting more of the
matter of heat into its interstices, and the vapour, which was
previously dissolved in this heat, is deposited, as is seen in the
receiver of an air-pump, which becomes dewy, as the air within becomes
expanded by the eduction of part of it. See note VII. Hence when the
mercury in the barometer sinks without a change of the wind the air
generally becomes colder. See note VII. on Elementary Heat. And it is
probably from the varying pressure of the incumbent air that in summer
days small black clouds are often thus suddenly produced, and again soon
vanish. See a paper in Philos. Trans. Vol. LXXVIII. intitled Frigorific
Experiments on the Mechanical Expansion of Air.
4. Another portion of atmospheric water may possibly be held in solution
by the electric fluid, since in thunder storms a precipitation of the
water seems to be either the cause or the consequence of the eduction of
the electricity. But it appears more probable that the water is
condensed into clouds by the eduction of its heat, and that then the
surplus of electricity prevents their coalescence into larger drops,
which immediately succeeds the departure of the lightning.
5. The immediate cause why the barometer sinks before rain is, first,
because a region of warm air, brought to us in the place of the cold air
which it had displaced, must weigh lighter, both specifically and
absolutely, if the height of the warm atmosphere be supposed to be equal
to that of the preceeding cold one. And secondly, after the drops of
rain begin to fall in any column of air, that column becomes lighter,
the falling drops only adding to the pressure of the air in proportion
to the resistance which they meet with in passing through that fluid.
If we could suppose water to be dissolved in air without heat, or in
very low degrees of heat, I suppose the air would become heavier, as
happens in many chemical solutions, but if water dissolved in the matter
of heat, or calorique, be mixed with an aerial solution of water, there
can be no doubt but an atmosphere consisting of such a mixture must
become lighter in proportion to the quantity of calorique. On the same
circumstance depends the visible vapour produced from the breath of
animals in cold weather, or from a boiling kettle; the particles of cold
air, with which it is mixed, steal a part of its heat, and become
themselves raised in temperature, whence part of the water is
precipitated in visible vapour, which, if in great quantity sinks to the
ground; if in small quantity, and the surrounding air is not previously
saturated, it spreads itself till it becomes again dissolved.
NOTE XXVI.--SPRINGS
_Your lucid bands condense with fingers chill
The blue mist hovering round the gelid hill_.
CANTO III. l. 19.
The surface of the earth consists of strata many of which were formed
originally beneath the sea, the mountains were afterwards forced up by
subterraneous fires, as appears from the fissures in the rocks of which
they consist, the quantity of volcanic productions all over the world,
and the numerous remains of craters of volcanos in mountainous
countries. Hence the strata which compose the sides of mountains lie
slanting downwards, and one or two or more of the external strata not
reaching to the summit when the mountain was raised up, the second or
third stratum or a more inferior one is there exposed to day; this may
be well represented by forceably thrusting a blunt instrument through
several sheets of paper, a bur will stand up with the lowermost sheet
standing highest in the center of it. On this uppermost stratum, which
is colder as it is more elevated, the dews are condensed in large
quantities; and sliding down pass under the first or second or third
stratum which compose the sides of the hill; and either form a morass
below, or a weeping rock, by oozing out in numerous places, or many of
these less currents meeting together burst out in a more copious rill.
The summits of mountains are much colder than the plains in their
vicinity, owing to several causes; 1. Their being in a manner insulated
or cut off from the common heat of the earth, which is always of 48
degrees, and perpetually counteracts the effects of external cold
beneath that degree. 2. From their surfaces being larger in proportion
to their solid contents, and hence their heat more expeditiously carried
away by the ever-moving atmosphere. 3. The increasing rarity of the air
as the mountain rises. All those bodies which conduct electricity well
or ill, conduct the matter of heat likewise well or ill. See note VII.
Atmospheric air is a bad conductor of electricity and thence confines it
on the body where it is accumulated, but when it is made very rare, as
in the exhausted receiver, the electric aura passes away immediately to
any distance. The same circumstance probably happens in respect to heat,
which is thus kept by the denser air on the plains from escaping, but is
dissipated on the hills where the air is thinner. 4. As the currents of
air rise up the sides of mountains they become mechanically rarefied,
the pressure of the incumbent column lessening as they ascend. Hence the
expanding air absorbs heat from the mountain as it ascends, as explained
in note VII. 5. There is another, and perhaps more powerful cause, I
suspect, which may occasion the great cold on mountains, and in the
higher parts of the atmosphere, and which has not yet been attended to;
I mean that the fluid matter of heat may probably gravitate round the
earth, and form an atmosphere on its surface, mixed with the aerial
atmosphere, which may diminish or become rarer, as it recedes from the
earth's surface, in a greater proportion than the air diminishes.
6. The great condensation of moisture on the summits of hills has
another cause, which is the dashing of moving clouds against them, in
misty days this is often seen to have great effect on plains, where an
eminent tree by obstructing the mist as it moves along shall have a much
greater quantity of moisture drop from its leaves than falls at the same
time on the ground in its vicinity. Mr. White, in his History of
Selborne gives an account of a large tree so situated, from which a
stream flowed during a moving mist so as to fill the cart-ruts in a lane
otherwise not very moist, and ingeniously adds, that trees planted about
ponds of stagnant water contribute much by these means to supply the
reservoir. The spherules which constitute a mist or cloud are kept from
uniting by so small a power that a little agitation against the leaves
of a tree, or the greater attraction of a flat moist surface, condenses
or precipitates them.
If a leaf has its surface moistened and particles of water separate from
each other as in a mist be brought near the moistened surface of a leaf,
each particle will be attracted more by that plain surface of water on
the leaf than it can be by the surrounding particles of the mist,
because globules only attract each other in one point, whereas a plain
attracts a globule by a greater extent of its surface.
The common cold springs are thus formed on elevated grounds by the
condensed vapours, and hence are stronger when the nights are cold after
hot days in spring, than even in the wet days of winter. For the warm
atmosphere during the day has dissolved much more water than it can
support in solution during the cold of the night, which is thus
deposited in large quantities on the hills, and yet so gradually as to
soak in between the strata of them, rather than to slide off over their
surfaces like showers of rain. The common heat of the internal parts of
the earth is ascertained by springs which arise from strata of earth too
deep to be affected by the heat of summer or the frosts of winter. Those
in this country are of 48 degrees of heat, those about Philidelphia were
said by Dr. Franklin to be 52; whether this variation is to be accounted
for by the difference of the sun's heat on that country, according to
the ingenious theory of Mr. Kirwan, or to the vicinity of subterranean
fires is not yet, I think, decided. There are however subterraneous
streams of water not exactly produced in this manner, as streams issuing
from fissures in the earth, communicating with the craters of old
volcanoes; in the Peak of Derbyshire are many hollows, called swallows,
where the land floods sink into the earth, and come out at some miles
distant, as at Ilam near Ashborne. See note on Fica, Vol. II.
Other streams of cold water arise from beneath the snow on the Alps and
Andes, and other high mountains, which is perpetualy thawing at its
under surface by the common heat of the earth, and gives rise to large
rivers. For the origin of warm springs see note on Fucus, Vol. II.
NOTE XXVII.--SHELL FISH.
_You round Echinus ray his arrowy mail,
Give the keel'd Nautilus his oar and sail.
Firm to his rock with silver cords suspend
The anchored Pinna, and his Cancer-friend_.
CANTO III. l. 67.
The armour of the Echinus, or Sea-hedge Hog, consists generally of
moveable spines; (_Linnei System. Nat._ Vol. I. p. 1102.) and in that
respect resembles the armour of the land animal of the same name. The
irregular protuberances on other sea-shells, as on some species of the
Purpura, and Murex, serve them as a fortification against the attacks of
their enemies.
It is said that this animal foresees tempestuous weather, and sinking to
the bottom of the sea adheres firmly to sea-plants, or other bodies by
means of a substance which resembles the horns of snails. Above twelve
hundred of these fillets have been counted by which this animal fixes
itself; and when afloat, it contracts these fillets between the bases of
its points, the number of which often amounts to two thousand. Dict
raisonne. art. Oursin. de mer.
There is a kind of Nautilus, called by Linneus, Argonauta, whose shell
has but one cell; of this animal Pliny affirms, that having exonerated
its shell by throwing out the water, it swims upon the surface,
extending a web of wonderful tenuity, and bending back two of its arms
and rowing with the rest, makes a sail, and at length receiving the
water dives again. Plin. IX. 29. Linneus adds to his description of this
animal, that like the Crab Diogenes or Bernhard, it occupies a house
not its own, as it is not connected to its shell, and is therefore
foreign to it; who could have given credit to this if it had not been
attested by so many who have with their own eyes seen this argonaut in
the act of sailing? Syst. Nat p. 1161.
The Nautilus, properly so named by Linneus, has a shell consisting of
many chambers, of which cups are made in the East with beautiful
painting and carving on the mother-pearl. The animal is said to inhabit
only the uppermost or open chamber, which is larger than the rest; and
that the rest remain empty except that the pipe, or siphunculus, which
communicates from one to the other of them is filled with an appendage
of the animal like a gut or string. Mr. Hook in his Philos. Exper. p.
306, imagines this to be a dilatable or compressible tube, like the air-
bladders of fish, and that by contracting or permitting it to expand, it
renders its shell boyant or the contrary. See Note on Ulva, Vol. II.
The Pinna, or Sea-wing, is contained in a two-valve shell, weighing
sometimes fifteen pounds, and emits a beard of fine long glossy silk-
like fibres, by which it is suspended to the rocks twenty or thirty feet
beneath the surface of the sea. In this situation it is so successfully
attacked by the eight-footed Polypus, that the species perhaps could not
exist but for the exertions of the Cancer Pinnotheris, who lives in the
same shell as a guard and companion. Amoen. Academ. Vol. II. p. 48. Lin.
Syst. Nat. Vol. I. p. 1159, and p. 1040.
The Pinnotheris, or Pinnophylax, is a small crab naked like Bernard the
Hermit, but is furnished with good eyes, and lives in the same shell
with the Pinna; when they want food the Pinna opens its shell, and sends
its faithful ally to forage; but if the Cancer sees the Polypus, he
returns suddenly to the arms of his blind hostess, who by closing the
shell avoids the fury of her enemy; otherwise, when it has procured a
booty, it brings it to the opening of the shell, where it is admitted,
and they divide the prey. This was observed by Haslequist in his voyage
to Palestine.
The Byssus of the antients, according to Aristotle, was the beard of the
Pinna above mentioned, but seems to have been used by other writers
indiscriminately for any spun material, which was esteemed finer or more
valuable than wool. Reaumur says the threads of this Byssus are not less
fine or less beautiful than the silk, as it is spun by the silk-worm;
the Pinna on the coasts of Italy and Provence (where it is fished up by
iron-hooks fixed on long poles) is called the silk-worm of the sea. The
stockings and gloves manufactured from it, are of exquisite fineness,
but too warm for common wear, and are thence esteemed useful in
rhumatism and gout. Dict. raisonné art. Pinne-marine. The warmth of the
Byssus, like that of silk, is probably owing to their being bad
conductors of heat, as well as of electricity. When these fibres are
broken by violence, this animal as well as the muscle has the power to
reproduce them like the common spiders, as was observed by M. Adanson.
As raw silk, and raw cobwebs, when swallowed, are liable to produce
great sickness (as I am informed) it is probable the part of muscles,
which sometimes disagrees with the people who eat them, may be this
silky web, by which they attach themselves to stones. The large kind of
Pinna contains some mother-pearl of a reddish tinge, according to M.
d'Argenville. The substance sold under the name of Indian weed, and used
at the bottom of fish-lines, is probably a production of this kind;
which however is scarcely to be distinguished by the eye from the
tendons of a rat's tail, after they have been separated by putrefaction
in water, and well cleaned and rubbed; a production, which I was once
shewn as a great curiosity; it had the uppermost bone of the tail
adhering to it, and was said to have been used as an ornament in a
lady's hair.
NOTE XXVIII.--STURGEON.
_With worm-like hard his toothless lips array,
And teach the unweildy Sturgeon to betray._
CANTO III. l. 71.
The Sturgeon, _Acipenser, Strurio._ Lin. Syst. Nat. Vol. I. p. 403. is a
fish of great curiosity as well as of great importance; his mouth is
placed under the head, without teeth, like the opening of a purse, which
he has the power to push suddenly out or retract. Before this mouth
under the beak or nose hang four tendrils some inches long, and which so
resemble earth-worms that at first sight they may be mistaken for them.
This clumsy toothless fish is supposed by this contrivance to keep
himself in good condition, the solidity of his flesh evidently shewing
him to be a fish of prey. He is said to hide his large body amongst the
weeds near the sea-coast, or at the mouths of large rivers, only
exposing his cirrhi or tendrils, which small fish or sea-insects
mistaking for real worms approach for plunder, and are sucked into the
jaws of their enemy. He has been supposed by some to root into the soil
at the bottom of the sea or rivers; but the cirrhi, or tendrills
abovementioned, which hang from his snout over his mouth, must
themselves be very inconvenient for this purpose, and as it has no jaws
it evidently lives by suction, and during its residence in the sea a
quantity of sea-insects are found in its stomach.
The flesh was so valued in the time of the Emperor Severus, that it was
brought to table by servants with coronets on their heads, and preceded
by music, which might give rise to its being in our country presented by
the Lord Mayor to the King. At present it is caught in the Danube, and
the Walga, the Don, and other large rivers for various purposes. The
skin makes the best covering for carriages; isinglass is prepared from
parts of the skin; cavear from the spawn; and the flesh is pickled or
salted, and sent all over Europe.
NOTE XXIX.--OIL ON WATER.
_Who with fine films, suspended o'er the deep,
Of Oil effusive lull the waves to sleep._
CANTO III. l. 87.
There is reason to believe that when oil is poured upon water, the two
surfaces do not touch each other, but that the oil is suspended over the
water by their mutual repulsion. This seems to be rendered probable by
the following experiment: if one drop of oil be droped on a bason of
water, it will immediately diffuse itself over the whole, for there
being no friction between the two surfaces, there is nothing to prevent
its spreading itself by the gravity of the upper part of it, except its
own tenacity, into a pellicle of the greatest tenuity. But if a second
drop of oil be put upon the former, it does not spread itself, but
remains in the form of a drop, as the other already occupied the whole
surface of the bason, and there is friction in oil passing over oil,
though none in oil passing over water.
Hence when oil is diffused on the surface of water gentle breezes have
no influence in raising waves upon it; for a small quantity of oil will
cover a very great surface of water, (I suppose a spoonful will diffuse
itself over some acres) and the wind blowing upon this carries it
gradually forwards; and there being no friction between the two surfaces
the water is not affected. On which account oil has no effect in
stilling the agitation of the water after the wind ceases, as was found
by the experiments of Dr. Franklin.
This circumstance lately brought into notice by Dr. Franklin had been
mentioned by Pliny, and is said to be in use by the divers for pearls,
who in windy weather take down with them a little oil in their mouths,
which they occasionally give out when the inequality of the supernatant
waves prevents them from seeing sufficiently distinctly for their
purpose.
The wonderful tenuity with which oil can be spread upon water is evinced
by a few drops projected from a bridge, where the eye is properly placed
over it, passing through all the prismatic colours as it diffuses
itself. And also from another curious experiment of Dr. Franklin's: he
cut a piece of cork to about the size of a letter-wafer, leaving a point
standing off like a tangent at one edge of the circle. This piece of
cork was then dipped in oil and thrown into a large pond of water, and
as the oil flowed off at the point, the cork-wafer continued to revolve
in a contrary direction for several minutes. The oil flowing off all
that time at the pointed tangent in coloured streams. In a small pond of
water this experiment does not so well succeed, as the circulation of
the cork stops as soon as the water becomes covered with the pellicle of
oil. See Additional Note, No. XIII. and Note on Fucus, Vol. II.
The ease with which oil and water slide over each other is agreeably
seen if a phial be about half filled with equal parts of oil and water,
and made to oscillate suspended by a string, the upper surface of the
oil and the lower one of the water will always keep smooth; but the
agitation of the surfaces where the oil and water meet, is curious; for
their specific gravities being not very different, and their friction on
each other nothing, the highest side of the water, as the phial descends
in its oscillation, having acquired a greater momentum than the lowest
side (from its having descended further) would rise the highest on the
ascending side of the oscillation, and thence pushes the then uppermost
part of the water amongst the oil.
NOTE XXX.--SHIP-WORM.
_Meet fell Teredo, as he mines the keel
With beaked head, and break his lips of steel._
CANTO III. l. 91.
The Teredo, or ship-worm, has two calcareous jaws, hemispherical, flat
before, and angular behind. The shell is taper, winding, penetrating
ships and submarine wood, and was brought from India into Europe, Linnei
System. Nat. p. 1267. The Tarieres, or sea-worms, attack and erode ships
with such fury, and in such numbers, as often greatly to endanger them.
It is said that our vessels have not known this new enemy above fifty
years, that they were brought from the sea about the Antilles to our
parts of the ocean, where they have increased prodigiously. They bore
their passage in the direction of the fibres of the wood, which is their
nourishment, and cannot return or pass obliquely, and thence when they
come to a knot in the wood, or when two of them meet together with their
stony mouths, they perish for want of food.
In the years 1731 and 1732 the United Provinces were under a dreadful
alarm concerning these insects, which had made great depredation on the
piles which support the banks of Zeland, but it was happily discovered a
few years afterwards that these insects had totally abandoned that
island, (Dict Raisonné, art, Vers Rongeurs,) which might have been
occasioned by their not being able to live in that latitude when the
winter was rather severer than usual.
NOTE XXXI.--MAELSTROM.
_Turn the broad helm, the fluttering canvas urge
From Maelstrom's fierce innavigable surge._
CANTO III. l. 93.
On the coast of Norway there is an extensive vortex, or eddy, which lies
between the islands of Moskoe and Moskenas, and is called Moskoestrom,
or Maelstrom; it occupies some leagues in circumference, and is said to
be very dangerous and often destructive to vessels navigating these
seas. It is not easy to understand the existence of a constant
descending stream without supposing it must pass through a subterranean
cavity to some other part of the earth or ocean which may lie beneath
its level; as the Mediterranean seems to lie beneath the level of the
Atlantic ocean, which therefore constantly flows into it through the
Straits; and the waters of the Gulph of Mexico lie much above the level
of the sea about the Floridas and further northward, which gives rise to
the Gulph-stream, as described in note on Cassia in Vol. II.
The Maelstrom is said to be still twice in about twenty-four hours when
the tide is up, and most violent at the opposite times of the day. This
is not difficult to account for, since when so much water is brought
over the subterraneous passage, if such exists, as compleatly to fill it
and stand many feet above it, less disturbance must appear on the
surface. The Maelstrom is described in the Memoires of the Swedish
Academy of Sciences, and Pontoppiden's Hist. of Norway, and in Universal
Museum for 1763, p. 131.
The reason why eddies of water become hollow in the middle is because
the water immediately over the centre of the well, or cavity, falls
faster, having less friction to oppose its descent, than the water over
the circumference or edges of the well. The circular motion or gyration
of eddies depends on the obliquity of the course of the stream, or to
the friction or opposition to it being greater on one side of the well
than the other; I have observed in water passing through a hole in the
bottom of a trough, which was always kept full, the gyration of the
stream might be turned either way by increasing the opposition of one
side of the eddy with ones finger, or by turning the spout, through
which the water was introduced, a little more obliquely to the hole on
one side or on the other. Lighter bodies are liable to be retained long
in eddies of water, while those rather heavier than water are soon
thrown out beyond the circumference by their acquired momentum becoming
greater than that of the water. Thus if equal portions of oil and water
be put into a phial, and by means of a string be whirled in a circle
round the hand, the water will always keep at the greater distance from
the centre, whence in the eddies formed in rivers during a flood a
person who endeavours to keep above water or to swim is liable to be
detained in them, but on suffering himself to sink or dive he is said
readily to escape. This circulation of water in descending through a
hole in a vessel Dr. Franklin has ingeniously applied to the explanation
of hurricanes or eddies of air.
NOTE XXXII.--GLACIERS.
_While round dark crags imprison'd waters bend
Through rifted ice, in ivory veins descend._
CANTO III. l. 113.
The common heat of the interior parts of the earth being always 48
degrees, both in winter and summer, the snow which lies in contact with
it is always in a thawing state; Hence in ice-houses the external parts
of the collection of ice is perpetually thawing and thus preserves the
internal part of it; so that it is necessary to lay up many tons for the
preservation of one ton. Hence in Italy considerable rivers have their
source from beneath the eternal glaciers, or mountains of snow and ice.
In our country when the air in the course of a frost continues a day or
two at very near 32 degrees, the common heat of the earth thaws the ice
on its surface, while the thermometer remains at the freezing point.
This circumstance is often observable in the rimy mornings of spring;
the thermometer shall continue at the freezing point, yet all the rime
will vanish, except that which happens to lie on a bridge, a board, or
on a cake of cow-dung, which being thus as it were insulated or cut off
from so free a communication with the common heat of the earth by means
of the air under the bridge, or wood, or dung, which are bad conductors
of heat, continues some time longer unthawed. Hence when the ground is
covered thick with snow, though the frost continues, and the sun does
not shine, yet the snow is observed to decrease very sensibly. For the
common heat of the earth melts the under surface of it, and the upper
one evaporates by its solution in the air. The great evaporation of ice
was observed by Mr. Boyle, which experiment I repeated some time ago.
Having suspended a piece of ice by a wire and weighed it with care
without touching it with my hand, I hung it out the whole of a clear
frosty night, and found in the morning it had lost nearly a fifth of its
weight. Mr. N. Wallerius has since observed that ice at the time of its
congelation evaporates faster than water in its fluid form; which may be
accounted for from the heat given out at the instant of freezing;
(Saussure's Essais sur Hygromet. p. 249.) but this effect is only
momentary.
Thus the vegetables that are covered with snow are seldom injured;
since, as they lie between the thawing snow, which has 32 degrees of
heat, and the covered earth which has 48, they are preserved in a degree
of heat between these; viz. in 40 degrees of heat. Whence the moss on
which the rein-deer feed in the northern latitudes vegetates beneath the
snow; (See note on Muschus, Vol. II.) and hence many Lapland and Alpine
plants perished through cold in the botanic garden at Upsal, for in
their native situations, though the cold is much more intense, yet at
its very commencement they are covered deep with snow, which remains
till late in the spring. For this fact see Amaenit. Academ. Vol. I. No.
48. In our climate such plants do well covered with dried fern, under
which they will grow, and even flower, till the severe vernal frosts
cease. For the increase of glaciers see Note on Canto I. l. 529.
NOTE XXXIII.--WINDS.
_While southern gales o'er western oceans roll,
And Eurus steals his ice-winds from the pole._
CANTO IV. l. 15.
The theory of the winds is yet very imperfect, in part perhaps owing to
the want of observations sufficiently numerous of the exact times and
places where they begin and cease to blow, but chiefly to our yet
imperfect knowledge of the means by which great regions of air are
either suddenly produced or suddenly destroyed.
The air is perpetually subject to increase or diminution from its
combination with other bodies, or its evolution from them. The vital
part of the air, called oxygene, is continually produced in this climate
from the perspiration of vegetables in the sunshine, and probably from
the action of light on clouds or on water in the tropical climates,
where the sun has greater power, and may exert some yet unknown laws of
luminous combination. Another part of the atmosphere, which is called
azote, is perpetually set at liberty from animal and vegetable bodies by
putrefaction or combustion, from many springs of water, from volatile
alcali, and probably from fixed alcali, of which there is an exhaustless
source in the water of the ocean. Both these component parts of the air
are perpetually again diminished by their contact with the soil, which
covers the surface of the earth, producing nitre. The oxygene is
diminished in the production of all acids, of which the carbonic and
muriatic exist in great abundance. The azote is diminished in the growth
of animal bodies, of which it constitutes an important part, and in its
combinations with many other natural productions.
They are both probably diminished in immense quantities by uniting with
the inflammable air, which arises from the mud of rivers and lakes at
some seasons, when the atmosphere is light: the oxygene of the air
producing water, and the azote producing volatile alcali by their
combinations with this inflammable air. At other seasons of the year
these principles may again change their combinations, and the
atmospheric air be reproduced.
Mr. Lavoisier found that one pound of charcoal in burning consumed two
pounds nine ounces of vital air, or oxygene. The consumption of vital
air in the process of making red lead may readily be reduced to
calculation; a small barrel contains about twelve hundred weight of this
commodity, 1200 pounds of lead by calcination absorb about 144 pounds of
vital air; now as a cubic foot of water weighs 1000 averdupois ounces,
and as vital air is above 800 times lighter than water, it follows that
every barrel of red lead contains nearly 2000 cubic feet of vital air.
If this can be performed in miniature in a small oven, what may not be
done in the immense elaboratories of nature!
These great elaboratories of nature include almost all her fossil as
well as her animal and vegetable productions. Dr. Priestley obtained air
of greater or less purity, both vital and azotic, from almost all the
fossil substances he subjected to experiment. Four ounce-weight of lava
from Iceland heated in an earthen retort yielded twenty ounce-measures
of air.
4 ounce-weight of lava gave 20 ounce measures of air.
7 ............... basaltes .... 104 ......................
2 ............... toadstone .... 40 ......................
11/2 ............... granite .... 20 ......................
1 ............... elvain .... 30 ......................
7 ............... gypsum .... 230 ......................
4 ............... blue slate .... 230 ......................
4 ............... clay .... 20 ......................
4 ............... limestone-spar .... 830 ......................
5 ............... limestone .... 1160 ......................
3 ............... chalk .... 630 ......................
31/2 ............... white iron-ore .... 560 ......................
4 ............... dark iron-ore .... 410 ......................
1/2 ............... molybdena .... 25 ......................
1/2 ............... stream tin .... 20 ......................
2 ............... steatites .... 40 ......................
2 ............... barytes .... 26 ......................
2 ............... black wad .... 80 ......................
4 ............... sand stone .... 75 ......................
3 ............... coal .... 700 ......................
In this account the fixed air was previously extracted from the
limestones by acids, and the heat applied was much less than was
necessary to extract all the air from the bodies employed. Add to this
the known quantities of air which are combined with the calciform ores,
as the ochres of iron, manganese, calamy, grey ore of lead, and some
idea may be formed of the great production of air in volcanic eruptions,
as mentioned in note on Chunda, Vol. II. and of the perpetual
absorptions and evolutions of whole oceans of air from every part of the
earth.
But there would seem to be an officina aeris, a shop where air is both
manufactured and destroyed in the greatest abundance within the polar
circles, as will hereafter be spoken of. Can this be effected by some
yet unknown law of the congelation of aqueous or saline fluids, which
may set at liberty their combined heat, and convert a part both of the
acid and alcali of sea-water into their component airs? Or on the
contrary can the electricity of the northern lights convert inflammable
air and oxygene into water, whilst the great degree of cold at the poles
unites the azote with some other base? Another officina aeris, or
manufacture of air, would seem to exist within the tropics or at the
line, though in a much less quantity than at the poles, owing perhaps to
the action of the sun's light on the moisture suspended in the air, as
will also be spoken of hereafter; but in all other parts of the earth
these absorptions and evolutions of air in a greater or less degree are
perpetually going on in inconceivable abundance; increased probably, and
diminished at different seasons of the year by the approach or
retrocession of the sun's light; future discoveries must elucidate this
part of the subject. To this should be added that as heat and
electricity, and perhaps magnetism, are known to displace air, that it
is not impossible but that the increased or diminished quantities of
these fluids diffused in the atmosphere may increase its weight a well
as its bulk; since their specific attractions or affinities to matter
are very strong, they probably also possess general gravitation to the
earth; a subject which wants further investigation. See Note XXVI.
SOUTH-WEST WINDS.
The velocity of the surface of the earth in moving round its axis
diminishes from the equator to the poles. Whence if a region of air in
this country should be suddenly removed a few degrees towards the north
it must constitute a western wind, because from the velocity it had
previously acquired in this climate by its friction with the earth it
would for a time move quicker than the surface of the country it was
removed to; the contrary must ensue when a region of air is transported
from this country a few degrees southward, because the velocity it had
acquired in this climate would be less than that of the earth's surface
where it was removed to, whence it would appear to constitute a wind
from the east, while in reality the eminent parts of the earth would be
carried against the too slow air. But if this transportation of air from
south to north be performed gradually, the motion of the wind will blow
in the diagonal between south and west. And on the contrary if a region
of air be gradually removed from north to south it would also blow
diagonally between the north and east, from whence we may safely
conclude that all our winds in this country which blow from the north or
east, or any point between them, consist of regions of air brought from
the north; and that all our winds blowing from the south or west, or
from any point between them, are regions of air brought from the south.
It frequently happens during the vernal months that after a north-east
wind has passed over us for several weeks, during which time the
barometer has flood at above 301/2 inches, it becomes suddenly succeeded
by a south-west wind, which also continues several weeks, and the
barometer sinks to nearly 281/2 inches. Now as two inches of the mercury
in the barometer balance one-fifteenth part of the whole atmosphere, an
important question here presents itself, _what is become of all this
air_.
1. This great quantity of air can not be carried in a superior current
towards the line, while the inferior current slows towards the poles,
because then it would equally affect the barometer, which should not
therefore subside from 301/2 inches to 281/2 for six weeks together.
2. It cannot be owing to the air having lost all the moisture which was
previously dissolved in it, because these warm south-west winds are
replete with moisture, and the cold north-east winds, which weigh up the
mercury in the barometer to 31 inches, consist of dry air.
3. It can not be carried over the polar regions and be accumulated on
the meridian, opposite to us in its passage towards the line, as such an
accumulation would equal one-fifteenth of the whole atmosphere, and can
not be supposed to remain in that situation for six weeks together.
4. It can not depend on the existence of tides in the atmosphere, since
it must then correspond to lunar periods. Nor to accumulations of air
from the specific levity of the upper regions of the atmosphere, since
its degree of fluidity must correspond with its tenuity, and
consequently such great mountains of air can not be supposed to exist
for so many weeks together as the south west winds sometimes continue.
5. It remains therefore that there must be at this time a great and
sudden absorption of air in the polar circle by some unknown operation
of nature, and that the south wind runs in to supply the deficiency. Now
as this south wind consists of air brought from a part of the earth's
surface which moves faster than it does in this climate it must have at
the same time a direction from the west by retaining part of the
velocity it had previously acquired. These south-west winds coming from
a warmer country, and becoming colder by their contact with the earth of
this climate, and by their expansion, (so great a part of the
superincumbent atmosphere having vanished,) precipitate their moisture;
and as they continue for several weeks to be absorbed in the polar
circle would seem to receive a perpetual supply from the tropical
regions, especially over the line, as will hereafter be spoken of.
It may sometimes happen that a north-east wind having passed over us may
be bent down and driven back before it has acquired any heat from the
climate, and may thus for a few hours or a day have a south-west
direction, and from its descending from a higher region of the
atmosphere may possess a greater degree of cold than an inferior north
east current of air.
The extreme cold of Jan. 13, 1709, at Paris came on with a gentle south
wind, and was diminished when the wind changed to the north, which is
accounted for by Mr. Homberg from a reflux of air which had been flowing
for some time from the north. Chemical Essays by R. Watson, Vol. V. p.
182.
It may happen that a north-east current may for a day or two pass over
us and produce incessant rain by mixing with the inferior south-west
current; but this as well as the former is of short duration, as its
friction will soon carry the inferior current along with it, and dry or
frosty weather will then succeed.
NORTH-EAST WINDS.
The north-east winds of this country consist of regions of air from the
north, travelling sometimes at the rate of about a mile in two minutes
during the vernal months for several weeks together from the polar
regions toward the south, the mercury in the barometer standing above
30. These winds consist of air greatly cooled by the evaporation of the
ice and snow over which it passes, and as they become warmer by their
contact with the earth of this climate are capable of dissolving more
moisture as they pass along, and are thence attended with frosts in
winter and with dry hot weather in summer.
1. This great quantity of air can not be supplied by superior currents
passing in a contrary direction from south to north, because such
currents must as they arise into the atmosphere a mile or two high
become exposed to so great cold as to occasion them to deposit their
moisture, which would fall through the inferior current upon the earth
in some part of their passage.
2. The whole atmosphere must have increased in quantity, because it
appears by the barometer that there exists one-fifteenth part more air
over us for many weeks together, which could not be thus accumulated by
difference of temperature in respect to heat, or by any aerostatic laws
at present known, or by any lunar influence.
From whence it would appear that immense masses of air were set at
liberty from their combinations with solid bodies, along with a
sufficient quantity of combined heat, within the polar circle, or in
some region to the north of us; and that they thus perpetually increase
the quantity of the atmosphere; and that this is again at certain times
re-absorbed, or enters into new combinations at the line or tropical
regions. By which wonderful contrivance the atmosphere is perpetually
renewed and rendered fit for the support of animal and vegetable life.
SOUTH-EAST WINDS.
The south-east winds of this country consist of air from the north which
had passed by us, or over us, and before it had obtained the velocity of
the earth's surface in this climate had been driven back, owing to a
deficiency of air now commencing at the polar regions. Hence these are
generally dry or freezing winds, and if they succeed north-east winds
should prognosticate a change of wind from north-east to south-west; the
barometer is generally about 30. They are sometimes attended with cloudy
weather, or rain, owing to their having acquired an increased degree of
warmth and moisture before they became retrograde; or to their being
mixed with air from the south.
2. Sometimes these south-east winds consist of a vertical eddy of north-
east air, without any mixture of south-west air; in that case the
barometer continues above 30, and the weather is dry or frosty for four
or five days together.
It should here be observed, that air being an elastic fluid must be more
liable to eddies than water, and that these eddies must extend into
cylinders or vortexes of greater diameter, and that if a vertical eddy
of north-east air be of small diameter or has passed but a little way to
the south of us before its return, it will not have gained the velocity
of the earth's surface to the south of us, and will in consequence
become a south-east wind.--But if the vertical eddy be of large
diameter, or has passed much to the south of us, it will have acquired
velocity from its friction with the earth's surface to the south of us,
and will in consequence on its return become a south-west wind,
producing great cold.
NORTH-WEST WINDS.
There seem to be three sources of the north-west winds of this
hemisphere of the earth. 1. When a portion of southern air, which was
passing over us, is driven back by accumulation of new air in the polar
regions. In this case I suppose they are generally moist or rainy winds,
with the barometer under 30, and if the wind had previously been in the
south-west, it would seem to prognosticate a change to the north-east.
2. If a current of north wind is passing over us but a few miles high,
without any easterly direction; and is bent down upon us, it must
immediately possess a westerly direction, because it will now move
faster than the surface of the earth where it arrives; and thus becomes
changed from a north-east to a north-west wind. This descent of a north-
east current of air producing a north-west wind may continue some days
with clear or freezing weather, as it may be simply owing to a vertical
eddy of north-east air, as will be spoken of below. It may otherwise be
forced down by a current of south-west wind passing over it, and in this
case it will be attended with rain for a few days by the mixture of the
two airs of different degrees of heat; and will prognosticate a change
of wind from north-east to south-west if the wind was previously in the
north-east quarter.
3. On the eastern coast of North America the north-west winds bring
frost, as the north-east winds do in this country, as appears from
variety of testimony. This seems to happen from a vertical spiral eddy
made in the atmosphere between the shore and the ridge of mountains
which form the spine or back-bone of that continent. If a current of
water runs along the hypothenuse of a triangle an eddy will be made in
the included angle, which will turn round like a water-wheel as the
stream passes in contact with one edge of it. The same must happen when
a sheet of air flowing along from the north-east rises from the shore in
a straight line to the summit of the Apalachian mountains, a part of the
stream of north-east air will flow over the mountains, another part will
revert and circulate spirally between the summit of the country and the
eastern shore, continuing to move toward the south; and thus be changed
from a north-east to a north-west wind.
This vertical spiral eddy having been in contact with the cold summits
of these mountains, and descending from higher parts of the atmosphere
will lose part of its heat, and thus constitute one cause of the greater
coldness of the eastern sides of North America than of the European
shores opposite to them, which is said to be equal to twelve degrees of
north latitude, which is a wonderful fact, not otherwise easy to be
explained, since the heat of the springs at Philadelphia is said to be
50, which is greater than the medium heat of the earth in this country.
The existence of vertical eddies, or great cylinders of air rolling on
the surface of the earth, is agreeable to the observations of the
constructors of windmills; who on this idea place the area of the sails
leaning backwards, inclined to the horizon; and believe that then they
have greater power than when they are placed quite perpendicularly. The
same kind of rolling cylinders of water obtain in rivers owing to the
friction of the water against the earth at their bottoms; as is known by
bodies having been observed to float upon their surfaces quicker than
when immersed to a certain depth. These vertical eddies of air probably
exist all over the earth's surface, but particularly at the bottom or
sides of mountains; and more so probably in the course of the south-west
than of the north-east winds; because the former fall from an eminence,
as it were, on a part of the earth where there is a deficiency of the
quantity of air; as is shewn by the sinking of the barometer: whereas
the latter are pushed or squeezed forward by an addition to the
atmosphere behind them, as appears by the rising of the barometer.
TRADE-WINDS.
A column of heated air becomes lighter than before, and will therefore
ascend, by the pressure of the cold air which surrounds it, like a cork
in water, or like heated smoke in a chimney.
Now as the sun passes twice over the equator for once over either
tropic, the equator has not time to become cool; and on this account it
is in general hotter at the line than at the tropics; and therefore the
air over the line, except in some few instances hereafter to be
mentioned, continues to ascend at all seasons of the year, pressed
upwards by regions of air brought from the tropics.
This air thus brought from the tropics to the equator, would constitute
a north wind on one side of the equator, and a south wind on the other;
but as the surface of the earth at the equator moves quicker than the
surface of the earth at the tropics, it is evident that a region of air
brought from either tropic to the equator, and which had previously only
acquired the velocity of the earth's surface at the tropics, will now
move too slow for the earth's surface at the equator, and will thence
appear to move in a direction contrary to the motion of the earth. Hence
the trade-winds, though they consist of regions of air brought from the
north on one side of the line, and from the south on the other, will
appear to have the diagonal direction of north-east and south-west
winds.
Now it is commonly believed that there are superior currents of air
passing over these north-east and south-west currents in a contrary
direction, and which descending near the tropics produce vertical
whirlpools of air. An important question here again presents itself,
_What becomes of the moisture which this heated air ought to deposit, as
it cools in the upper regions of the atmosphere in its journey to the
tropics?_ It has been shewn by Dr. Priestley and Mr. Ingenhouz that the
green matter at the bottom of cisterns, and the fresh leaves of plants
immersed in water, give out considerable quantities of vital air in the
sun-shine; that is, the perspirable matter of plants (which is water
much divided in its egress from their minute pores) becomes decomposed
by the sun's light, and converted into two kinds of air, the vital and
inflammable airs. The moisture contained or dissolved in the ascending
heated air at the line must exist in great tenuity; and by being exposed
to the great light of the sun in that climate, the water may be
decomposed, and the new airs spread on the atmosphere from the line to
the poles.
1. From there being no constant deposition of rains in the usual course
of the trade-winds, it would appear that the water rising at the line is
decomposed in its ascent.
2. From the observations of M. Bougner on the mountain Pinchinca, one of
the Cordelieres immediately under the line, there appears to be no
condensible vapour above three or four miles high. Now though the
atmosphere at that height may be cold to a very considerable degree; yet
its total deprivation of condensible vapour would seem to shew, that its
water was decomposed; as there are no experiments to evince that any
degree of cold hitherto known has been able to deprive air of its
moisture; and great abundance of snow is deposited from the air that
flows to the polar regions, though it is exposed to no greater degrees
of cold in its journey thither than probably exists at four miles height
in the atmosphere at the line.
3. The hygrometer of Mr. Sauffure also pointed to dryness as he ascended
into rarer air; the single hair of which it was constructed, contracting
from deficiency of moisture. Essais sur l'Hygromet. p. 143.
From these observations it appears either that rare and cold air
requires more moisture to saturate it than dense air; or that the
moisture becomes decomposed and converted into air, as it ascends into
these cold and rare regions of the atmosphere.
4. There seems some analogy between the circumstance of air being
produced or generated in the cold parts of the atmosphere both at the
line and at the poles.
MONSOONS AND TORNADOES.
1. In the Arabian and Indian seas are winds, which blow six months one
way, and six months the other, and are called Monsoons; by the
accidental dispositions of land and sea it happens, that in some places
the air near the tropic is supposed to become warmer when the sun is
vertical over it, than at the line. The air in these places
consequently ascends pressed upon one side by the north-east regions of
air, and on the other side by the south-west regions of air. For as the
air brought from the south has previously obtained the velocity of the
earth's surface at the line, it moves faster than the earth's surface
near the tropic where it now arrives, and becomes a south-west wind,
while the air from the north becomes a north-east wind as before
explained. These two winds do not so quietly join and ascend as the
north-east and south-east winds, which meet at the line with equal
warmth and velocity and form the trade-winds; but as they meet in
contrary directions before they ascend, and cannot be supposed
accurately to balance each other, a rotatory motion will be produced as
they ascend like water falling through a hole, and an horizontal or
spiral eddy is the consequence; these eddies are more or less rapid, and
are called Tornadoes in their most violent state, raising water from the
ocean in the west or sand from the deserts of the east, in less violent
degrees they only mix together the two currents of north-east and south-
west air, and produce by this means incessant rains, as the air of the
north-east acquires some of the heat from the south-west wind, as
explained in Note XXV. This circumstance of the eddies produced by the
monsoon-winds was seen by Mr. Bruce in Abyssinia; he relates that for
many successive mornings at the commencement of the rainy monsoon, he
observed a cloud of apparently small dimensions whirling round with
great rapidity, and in few minutes the heavens became covered with dark
clouds with consequent great rains. See Note on Canto III. l. 129.
2. But it is not only at the place where the air ascends at the northern
extremity of the rainy monsoon, and where it forms tornadoes, as
observed above by Mr. Bruce, but over a great tract of country several
degrees in length in certain parts as in the Arabian sea, a perpetual
rain for several months descends, similar to what happens for weeks
together in our own climate in a less degree during the south-west
winds. Another important question presents itself here, _if the climate
to which this south-west wind arrives, it not colder than that it comes
from, why should it deposit its moisture during its whole journey? if it
be a colder climate, why does it come thither?_ The tornadoes of air
above described can extend but a little way, and it is not easy to
conceive that a superior cold current of air can mix with an inferior
one, and thus produce showers over ten degrees of country, since at
about three miles high there is perpetual frost; and what can induce
these narrow and shallow currents to flow over each other so many
hundred miles?
Though the earth at the northren extremity of this monsoon may be more
heated by certain circumstances of situation than at the line, yet it
seems probable that the intermediate country between that and the line,
may continue colder than the line (as in other parts of the earth) and
hence that the air coming from the line to supply this ascent or
destruction of air at the northern extremity of the monsoon will be
cooled all the way in its approach, and in consequence deposit its
water. It seems probable that at the northern extremity of this monsoon,
where the tornadoes or hurricanes exist, that the air not only ascends
but is in part converted into water, or otherwise diminished in
quantity, as no account is given of the existence of any superior
currents of it.
As the south-west winds are always attended with a light atmosphere, an
incipient vacancy, or a great diminution of air must have taken place to
the northward of them in all parts of the earth wherever they exist, and
a deposition of their moisture succeeds their being cooled by the
climate they arrive at, and not by a contrary current of cold air over
them, since in that case the barometer would not sink. They may thus in
our own country be termed monsoons without very regular periods.
3. Another cause of TORNADOES independent of the monsoons is ingeniously
explained by Dr. Franklin, when in the tropical countries a stratum of
inferior air becomes so heated by its contact with the warm earth, that
its expansion is increased more than is equivalent to the pressure of
the stratum of air over it; or when the superior stratum becomes more
condensed by cold than the inferior one by pressure, the upper region
will descend and the lower one ascend. In this situation if one part of
the atmosphere be hotter from some fortuitous circumstances, or, has
less pressure over it, the lower stratum will begin to ascend at this
part, and resemble water falling through a hole as mentioned above. If
the lower region of air was going forwards with considerable velocity,
it will gain an eddy by riling up this hole in the incumbent heavy air,
so that the whirlpool or tornado has not only its progressive velocity,
but its circular one also, which thus lifts up or overturns every thing
within its spiral whirl. By the weaker whirlwinds in this country the
trees are sometimes thrown down in a line of only twenty or forty yards
in breadth, making a kind of avenue through a country. In the West
Indies the sea rises like a cone in the whirl, and is met by black
clouds produced by the cold upper air and the warm lower air being
rapidly mixed; whence are produced the great and sudden rains called
water-spouts; while the upper and lower airs exchange their plus or
minus electricity in perpetual lightenings.
LAND AND SEA-BREEZES.
The sea being a transparent mass is less heated at its surface by the
sun's rays than the land, and its continual change of surface
contributes to preserve a greater uniformity in the heat of the air
which hangs over it. Hence the surface of the tropical islands is more
heated during the day than the sea that surrounds them, and cools more
in the night by its greater elevation: whence in the afternoon when the
lands of the tropical islands have been much heated by the sun, the air
over them ascends pressed upwards by the cooler air of the incircling
ocean, in the morning again the land becoming cooled more than the sea,
the air over it descends by its increased gravity, and blows over the
ocean near its shores.
CONCLUSION.
1. There are various irregular winds besides those above described,
which consist of horizontal or vertical eddies of air owing to the
inequality of the earth's surface, or the juxtaposition of the sea.
Other irregular winds have their origin from increased evaporation of
water, or its sudden devaporation and descent in showers; others from
the partial expansion and condensation of air by heat and cold; by the
accumulation or defect of electric fluid, or to the air's new production
or absorption occasioned by local causes not yet discovered. See Notes
VII. and XXV.
2. There seem to exist only two original winds: one consisting of air
brought from the north, and the other of air brought from the south. The
former of these winds has also generally an apparent direction from the
east, and the latter from the west, arising from the different
velocities of the earth's surface. All the other winds above described
are deflections or retrogressions of some parts of these currents of air
from the north or south.
3. One fifteenth part of the atmosphere is occasionally destroyed, and
occasionally reproduced by unknown causes. These causes are brought into
immediate activity over a great part of the surface of the earth at
nearly the same time, but always act more powerful to the northward than
to the southward of any given place; and would hence seem to have their
principal effect in the polar circles, existing nevertheless though with
less power toward the tropics or at the line.
For when the north-east wind blows the barometer rises, sometimes from
281/2 inches to 301/2, which shews a great new generation of air in the
north; and when the south-west wind blows the barometer sinks as much,
which shews a great destruction of air in the north. But as the north-
east winds sometimes continue for five or six weeks, the newly-generated
air must be destroyed at those times in the warmer climates to the south
of us, or circulate in superior currents, which has been shewn to be
improbable from its not depositing its water. And as the south-west
winds sometimes continue for some weeks, there must be a generation of
air to the south at those times, or superior currents, which last has
been shewn to be improbable.
4. The north-east winds being generated about the poles are pushed
forwards towards the tropics or line, by the pressure from behind, and
hence they become warmer, as explained in Note VII. as well as by their
coming into contact with a warmer part of the earth which contributes to
make these winds greedily absorb moisture in their passage. On the
contrary, the south-west winds, as the atmosphere is suddenly diminished
in the polar regions, are drawn as it were into an incipient vacancy,
and become therefore expanded in their passage, and thus generate cold,
as explained in Note VII. and are thus induced to part with their
moisture, as well as by their contact with a colder part of the earth's
surface. Add to this, that the difference in the sound of the north-east
and south-west winds may depend on the former being pushed forwards by a
pressure behind, and the latter falling as it were into a partial or
incipient vacancy before; whence the former becomes more condensed, and
the latter more rarefied as it passes. There is a whistle, termed a
lark-call, which consists of a hollow cylinder of tin-plate, closed at
each end, about half an inch in diameter and a quarter of an inch high,
with opposite holes about the size of a goose-quill through the centre
of each end; if this lark-whistle be held between the lips the sound of
it is manifestly different when the breath is forceably blown through it
from within outwards, and when it is sucked from without inwards.
Perhaps this might be worthy the attention of organ-builders.
5. A stop is put to this new generation of air, when about a fifteenth
of the whole is produced, by its increasing pressure; and a similar
boundary is fixed to its absorption or destruction by the decrease of
atmospheric pressure. As water requires more heat to convert it into
vapour under a heavy atmosphere than under a light one, so in letting
off the water from muddy fish-ponds great quantities of air-bubbles are
seen to ascend from the bottom, which were previously confined there by
the pressure of the water. Similar bubbles of inflammable air are seen
to arise from lakes in many seasons of the year, when the atmosphere
suddenly becomes light.
6. The increased absorptions and evolutions of air must, like its simple
expansions, depend much on the presence or absence of heat and light,
and will hence, in respect to the times and places of its production and
destruction, be governed by the approach or retrocession of the sun, and
on the temperature, in regard to heat, of various latitudes, and parts
of the same latitude, so well explained by Mr. Kirwan.
7. Though the immediate cause of the destruction or reproduction of
great masses of air at certain times, when the wind changes from north
to south, or from south to north can not yet be ascertained; yet as
there appears greater difficulty in accounting for this change of wind
for any other known causes, we may still suspect that there exists in
the arctic and antarctic circles a BEAR or DRAGON yet unknown to
philosophers, which at times suddenly drinks up, and as suddenly at
other times vomits out one-fifteenth part of the atmosphere: and hope
that this or some future age will learn how to govern and domesticate a
monster which might be rendered of such important service to mankind.
INSTRUMENTS.
If along with the usual registers of the weather observations were made
on the winds in many parts of the earth with the three following
instruments, which might be constructed at no great expence, some useful
information might be acquired.
1. To mark the hour when the wind changes from north-east to south-west,
and the contrary. This might be managed by making a communication from
the vane of a weathercock to a clock; in such a manner, that if the vane
mould revolve quite round, a tooth on its revolving axis should stop the
clock, or put back a small bolt on the edge of a wheel revolving once in
twenty-four hours.
2. To discover whether in a year more air passed from north to south, or
the contrary. This might be effected by placing a windmill-sail of
copper about nine inches diameter in a hollow cylinder about six inches
long, open at both ends, and fixed on an eminent situation exactly north
and south. Thence only a part of the north-east and south-west currents
would affect the sail so as to turn it; and if its revolutions were
counted by an adapted machinery, as the sail would turn one way with the
north currents of air, and the contrary one with the south currents, the
advance of the counting finger either way would shew which wind had
prevailed most at the end of the year.
3. To discover the rolling cylinders of air, the vane of a weathercock
might be so suspended as to dip or rise vertically, as well as to have
its horizontal rotation.
RECAPITULATION.
NORTH-EAST WINDS consist of air flowing from the north, where it seems
to be occasionally produced; has an apparent direction from the east
owing to its not having acquired in its journey the increasing velocity
of the earth's surface; these winds are analogous to the trade-winds
between the tropics, and frequently continue in the vernal months for
four and six weeks together, with a high barometer, and fair or frosty
weather. 2. They sometimes consist of south-west air, which had passed
by us or over us, driven back by a new accumulation of air in the north,
These continue but a day or two, and are attended with rain. See Note
XXV.
SOUTH-WEST WIND consists of air flowing from the south, and seems
occasionally absorbed at its arrival to the more northern latitudes. It
has a real direction from the west owing to its not having lost in its
journey the greater velocity it had acquired from the earth's surface
from whence it came. These winds are analogous to the monsoons between
the tropics, and frequently continue for four or six weeks together,
with a low barometer and rainy weather. 2. They sometimes consist of
north-east air, which had passed by us or over us, which becomes
retrograde by a commencing deficiency of air in the north. These winds
continue but a day or two, attended with severer frost with a sinking
barometer; their cold being increased by their expansion, as they
return, into an incipient vacancy.
NORTH-WEST WINDS consist, first, of south-west winds, which have passed
over us, bent down and driven back towards the south by newly generated
northern air. They continue but a day or two, and are attended with rain
or clouds. 2. They consist of north-east winds bent down from the higher
parts of the atmosphere, and having there acquired a greater velocity
than, the earth's surface; are frosty or fair. 3. They consist of north-
east winds formed into a vertical spiral eddy, as on the eastern coasts
of North America, and bring severe frost.
SOUTH-EAST WINDS consist, first, of north-east winds become retrograde,
continue for a day or two, frosty or fair, sinking barometer. 2. They
consist of north-east winds formed into a vertical eddy not a spiral
one, frost or fair.
NORTH WINDS consist, first, of air flowing slowly from the north, so
that they acquire the velocity of the earth's surface as they approach,
are fair or frosty, seldom occur. 2. They consist of retrograde south
winds; these continue but a day or two, are preceded by south-west
winds; and are generally succeeded by north-east winds, cloudy or rainy,
barometer rising.
SOUTH WINDS consist, first, of air flowing slowly from the south,
loosing their previous western velocity by the friction of the earth's
surface as they approach, moist, seldom occur, 2. They consist of
retrograde north winds; these continue but a day or two, are preceded by
north-east winds, and generally succeeded by south-west winds, colder,
barometer sinking.
EAST WINDS consist of air brought hastily from the north, and not
impelled farther southward, owing to a sudden beginning absorption of
air in the northern regions, very cold, barometer high, generally
succeeded by south-west wind.
WEST WINDS consist of air brought hastily from the south, and checked
from proceeding further to the north by a beginning production of air in
the northern regions, warm and moist, generally succeeded by north-east
wind. 2. They consist of air bent down from the higher regions of the
atmosphere, if this air be from the south, and brought hastily it
becomes a wind of great velocity, moving perhaps 60 miles an hour, is
warm and rainy; if it consists of northern air bent down it is of less
velocity and colder.
_Application of the preceding Theory to Some Extracts
from a Journal of the Weather._
_Dec. 1, 1790._ The barometer sunk suddenly, and the wind, which had
been some days north-east with frost, changed to south-east with an
incessant though moderate fall of snow. A part of the northern air,
which had passed by us I suppose, now became retrograde before it had
acquired the velocity of the earth's surface to the south of us, and
being attended by some of the southern air in its journey, the moisture
of the latter became condensed and frozen by its mixture mith the
former.
_Dec. 2, 3._ The wind changed to north-west and thawed the snow. A part
of the southern air, which had passed by us or over us, with the
retrograde northern air above described, was now in its turn driven
back, before it had lost the velocity of the surface of the earth to the
south of us, and consequently became a north-west wind; and not having
lost the warmth it brought from the south produced a thaw.
_Dec. 4, 5._ Wind changed to north-east with frost and a rising
barometer. The air from the north continuing to blow, after it had
driven back the southern air as above described, became a north-east
wind, having less velocity than the surface of the earth in this
climate, and produced frost from its coldness.
_Dec. 6, 7._ Wind now changed to the south-west with incessant rain and
a sinking barometer. From unknown causes I suppose the quantity of air
to be diminished in the polar regions, and the southern air cooled by
the earth's surface, which was previously frozen, deposits its moisture
for a day or two; afterwards the wind continued south-west without rain,
as the surface of the earth became warmer.
_March 18, 1785._ There has been a long frost; a few days ago the
barometer sunk to 291/2, and the frost became more severe. Because the air
being expanded by a part of the pressure being taken off became colder.
This day the mercury rose to 30, and the frost ceased, the wind
continuing as before between north and east. _March 19._ Mercury above
30, weather still milder, no frost, wind north-east. _March 20._ The
same, for the mercury rising shews that the air becomes more compressed
by the weight above, and in consequence gives out warmth.
_April 4, 5._ Frost, wind north-east, the wind changed in the middle of
the day to the north-west without rain, and has done so for three or
four days, becoming again north-east at night. For the sun now giving
greater degrees of heat, the air ascends as the sun passes the zenith,
and is supplied below by the air on the western side as well as on the
eastern side of the zenith during the hot part of the day; whence for a
few hours, on the approach of the hot part of the day, the air acquires
a westerly direction in this longitude. If the north-west wind had been
caused by a retrograde motion of some southern air, which had passed
over us, it would have been attended with rain or clouds.
_April 10._ It rained all day yesterday, the wind north-west, this
morning there was a sharp frost. The evaporation of the moisture, (which
fell yesterday) occasioned by the continuance of the wind, produced so
much cold as to freeze the dew.
_May 12._ Frequent showers with a current of colder wind preceding every
shower. The sinking of the rain or cloud pressed away the air from
beneath it in its descent, which having been for a time shaded from the
sun by the floating cloud, became cooled in some degree.
_June 20._ The barometer sunk, the wind became south-west, and the whole
heaven was instantly covered with clouds. A part of the incumbent
atmosphere having vanished, as appeared by the sinking of the barometer,
the remainder became expanded by its elasticity, and thence attracted
some of the matter of heat from the vapour intermixed with it, and thus
in a few minutes a total devaporation took place, as in exhausting the
receiver of an air-pump. See note XXV. At the place where the air is
destroyed, currents both from the north and south flow in to supply the
deficiency, (for it has been shewn that there are no other proper winds
but these two) and the mixture of these winds produces so sudden
condensation of the moisture, both by the coldness of the northern air
and the expansion of both of them, that lightning is given out, and an
incipient tornado takes place; whence thunder is said frequently to
approach against the wind.
_August 28, 1732._ Barometer was at 31, and _Dec. 30_, in the same year,
it was at 28 2-tenths. Medical Essays, Edinburgh, Vol. II. p. 7. It
appears from these journals that the mercury at Edinburgh varies
sometimes nearly three inches, or one tenth of the whole atmosphere.
From the journals kept by the Royal Society at London it appears seldom
to vary more than two inches, or one-fifteenth of the whole atmosphere.
The quantity of the variation is said still to decrease nearer the line,
and to increase in the more northern latitudes; which much confirms the
idea that there exists at certain times a great destruction or
production of air within the polar circle.
_July 2, 1732._ The westerly winds in the journal in the Medical Essays,
Vol. II. above referred to, are frequently marked with the number three
to shew their greater velocity, whereas the easterly winds seldom
approach to the number two. The greater velocity of the westerly winds
than the easterly ones is well known I believe in every climate of the
world; which may be thus explained from the theory above delivered. 1.
When the air is still, the higher parts of the atmosphere move quicker
than those parts which touch the earth, because they are at a greater
distance from the axis of motion. 2. The part of the atmosphere where
the north or south wind comes from is higher than the part of it where
it comes to, hence the more elevated parts of the atmosphere continue to
descend towards the earth as either of those winds approach. 3. When
southern air is brought to us it possesses a westerly direction also,
owing to the velocity it had previously acquired from the earth's
surface; and if it consists of air from the higher parts of the
atmosphere descending nearer the earth, this westerly velocity becomes
increased. But when northern air is brought to us, it possesses an
apparent easterly direction also, owing to the velocity which it had
previously acquired from the earth's surface being less than that of the
earth's surface in this latitude; now if the north-east wind consists of
air descending from higher parts of the atmosphere, this deficiency of
velocity will be less, in consequence of the same cause, viz. The higher
parts of the atmosphere descending, as the wind approaches, increases
the real velocity of the western winds, and decreases the apparent
velocity of the eastern ones.
_October 22._ Wind changed from south-east to south-west. There is a
popular prognostication that if the wind changes from the north towards
the south passing through the east, it is more likely to continue in the
south, than if it passes through the west, which may be thus accounted
for. If the north-east wind changes to a north-west wind, it shews
either that a part of the northern air descends upon us in a spiral
eddy, or that a superior current of southern air is driven back; but if
a north-east wind be changed into a south-east wind it shews that the
northern air is become retrograde, and that in a day or two, as soon as
that part of it has passed, which has not gained the velocity of the
earth's surface in this latitude, it will become a south wind for a few
hours, and then a south-west wind.
The writer of this imperfect sketch of anemology wishes it may incite
some person of greater leizure and ability to attend to this subject,
and by comparing the various meteorological journals and observations
already published, to construct a more accurate and methodical treatise
on this interesting branch of philosophy.
NOTE XXXIV.--VEGETABLE PERSPIRATION.
_And wed the enamoured Oxygene to Light._
CANTO IV. l. 34.
When points or hairs are put into spring-water, as in the experiments of
Sir B. Thompson, (Philos. Trans. Vol. LXXVII.) and exposed to the light
of the sun, much air, which loosely adhered to the water, rises in
bubbles, as explained in note on Fucus, Vol. II. A still greater
quantity of air, and of a purer kind, is emitted by Dr. Priestley's
green matter, and by vegetable leaves growing in water in the sun-shine,
according to Mr. Ingenhouze's experiments; both which I suspect to be
owing to a decomposition of the water perspired by the plant, for the
edge of a capillary tube of great tenuity may be considered as a circle
of points, and as the oxygene, or principle of vital air, may be
expanded into a gas by the sun's light; the hydrogene or inflammable air
may be detained in the pores of the vegetable.
Hence plants growing in the shade are white, and become green by being
exposed to the sun's light; for their natural colour being blue, the
addition of hydrogene adds yellow to this blue, and _tans_ them green. I
suppose a similar circumstance takes place in animal bodies; their
perspirable matter as it escapes in the sun-shine becomes decomposed by
the edges of their pores as in vegetables, though in less quantity, as
their perspiration is less, and by the hydrogene being retained the skin
becomes _tanned_ yellow. In proof of this it must be observed that both
vegetable and animal substances become bleached white by the sun-beams
when they are dead, as cabbage-stalks, bones, ivory, tallow, bees-wax,
linen and cotton cloth; and hence I suppose the copper-coloured natives
of sunny countries might become etiolated or blanched by being kept from
their infancy in the dark, or removed for a few generations to more
northerly climates.
It is probable that on a sunny morning much pure air becomes separated
from the dew by means of the points of vegetables on which it adheres,
and much inflammable air imbibed by the vegetable, or combined with it;
and by the sun's light thus decomposing water the effects of it in
bleaching linen seems to depend (as described in Note X.): the water is
decomposed by the light at the ends or points of the cotton or thread,
and the vital air unites with the phlogistic or colouring matters of the
cloth, and produces a new acid, which is either itself colourless or
washes out, at the same time the inflammable part of the water escapes.
Hence there seems a reason why cotton bleaches so much sooner than
linen, viz. because its fibres are three or four times shorter, and
therefore protrude so many more points, which seem to facilitate the
liberation of the vital air from the inflammable part of the water.
Bee's wax becomes bleached by exposure to the sun and dews in a similar
manner as metals become calcined or rusty, viz. by the water on their
surface being decomposed; and hence the inflammable material which
caused the colour becomes united with vital air forming a new acid, and
is washed away.
Oil close stopped in a phial not full, and exposed long to the sun's
light, becomes bleached, as I suppose, by the decomposition of the water
it contains; the inflammable air rising above the surface, and the vital
air uniting with the colouring matter of the oil. For it is remarkable,
that by shutting up a phial of bleached oil in a dark drawer, it in a
little time becomes coloured again.
The following experiment shews the power of light in separating vital
air from another basis, viz. from azote. Mr. Scheel inverted a glass
vessel filled with colourless nitrous acid into another glass containing
the same acid, and on exposing them to the sun's light, the inverted
glass became partly filled with pure air, and the acid at the same time
became coloured. Scheel in Crell's Annal. 1786. But if the vessel of
colourless nitrous acid be quite full and stopped, so that no space is
left for the air produced to expand itself into, no change of colour
takes place. Priestley's Exp. VI. p. 344. See Keir's very excellent
Chemical Dictionary, p. 99. new edition.
A sun-flower three feet and half high according to the experiment of Dr.
Hales, perspired two pints in one day (Vegetable Statics.) which is many
times as much in proportion to its surface, as is perspired from the
surface and lungs of animal bodies; it follows that the vital air
liberated from the surfaces of plants by the sunshine must much exceed
the quantity of it absorbed by their respiration, and that hence they
improve the air in which they live during the light part of the day, and
thus blanched vegetables will sooner become _tanned into green_ by the
sun's light, than etiolated animal bodies will become _tanned yellow_ by
the same means.
It is hence evident, that the curious discovery of Dr. Priestley, that
his green vegetable matter and other aquatic plants gave out vital air
when the sun shone upon them, and the leaves of other plants did the
same when immersed in water, as observed by Mr. Ingenhouze, refer to the
perspiration of vegetables not to their respiration. Because Dr.
Priestley observed the pure air to come from both sides of the leaves
and even from the stalks of a water-flag, whereas one side of the leaf
only serves the office of lungs, and certainly not the stalks. Exper. on
Air, Vol. III. And thus in respect to the circumstance in which plants
and animals seemed the furtherest removed from each other, I mean in
their supposed mode of respiration, by which one was believed to purify
the air which the other had injured, they seem to differ only in degree,
and the analogy between them remains unbroken.
Plants are said by many writers to grow much faster in the night than in
the day; as is particularly observable in seedlings at their rising out
of the ground. This probably is a consequence of their sleep rather than
of the absence of light; and in this I suppose they also resemble animal
bodies.
NOTE XXXV.--VEGETABLE PLACENTATION.
_While in bright veins the silvery sap ascends.
CANTO IV. l. 419.
As buds are the viviparous offspring of vegetables, it becomes necessary
that they should be furnished with placental vessels for their
nourishment, till they acquire lungs or leaves for the purpose of
elaborating the common juices of the earth into nutriment. These vessels
exist in bulbs and in seeds, and supply the young plant with a sweet
juice till it acquires leaves, as is seen in converting barley into
malt, and appears from the sweet taste of onions and potatoes, when they
begin to grow.
The placental vessels belonging to the buds of trees are placed about
the roots of most, as the vine; so many roots are furnished with sweet
or mealy matter as fern-root, bryony, carrot, turnip, potatoe, or in the
alburnum or sap-wood as in those trees which produce manna, which is
deposited about the month of August, or in the joints of sugar cane, and
grasses; early in the spring the absorbent mouths of these vessels drink
up moisture from the earth, with a saccharine matter lodged for that
purpose during the preceding autumn, and push this nutritive fluid up
the vessels of the alburnum to every individual bud, as is evinced by
the experiments of Dr. Hales, and of Mr. Walker in the Edinburgh
Philosophical Transact. The former observed that the sap from the stump
of a vine, which he had cut off in the beginning of April, arose twenty-
one feet high in tubes affixed to it for that purpose, but in a few
weeks it ceased to bleed at all, and Dr. Walker marked the progress of
the ascending sap, and found likewise that as soon as the leaves became
expanded the sap ceased to rise; the ascending juice of some trees is so
copious and so sweet during the sap-season that it is used to make wine,
as the birch, betula, and sycamore, acer pseudo-platinus, and
particularly the palm.
During this ascent of the sap-juice each individual leaf-bud expands its
new leaves, and shoots down new roots, covering by their intertexture
the old bark with a new one; and as soon as these new roots (or bark)
are capable of absorbing sufficient juices from the earth for the
support of each bud, and the new leaves are capable of performing their
office of exposing these juices to the influence of the air; the
placental vessels cease to act, coalesce, and are transformed from sap-
wood, or alburnum, into inert wood; serving only for the support of the
new tree, which grows over them.
Thus from the pith of the new bud of the horse-chesnut five vessels pass
out through the circle of the placental vessels above described, and
carry with them a minuter circle of those vessels; these five bundles of
vessels unite after their exit, and form the footstalk or petiole of the
new five-fingered leaf, to be spoken of hereafter. This structure is
well seen by cutting off a leaf of the horse-chesnut (Aesculus
Hippocastanum) in September before it falls, as the buds of this tree
are so large that the flower may be seen in them with the naked eye.
After a time, perhaps about midsummer, another bundle of vessels passes
from the pith through the alburnum or sap-vessels in the bosom of each
leaf, and unites by the new bark with the leaf, which becomes either a
flower-bud or a leaf-bud to be expanded in the ensuing spring, for which
purpose an apparatus of placental vessels are produced with proper
nutriment during the progress of the summer and autumn, and thus the
vegetable becomes annually increased, ten thousand buds often existing
on one tree, according to the estimate of Linneus. Phil. Bot.
The vascular connection of vegetable buds with the leaves in whose
bosoms they are formed is confirmed by the following experiment, (Oct.
20, 1781.) On the extremity of a young bud of the Mimosa (sensitive
plant) a small drop of acid of vitriol was put by means of a pen, and,
after a few seconds, the leaf in whose axilla it dwelt closed and opened
no more, though the drop of vitriolic acid was so small as apparently
only to injure the summit of the bud. Does not this seem to shew that
the leaf and its bud have connecting vessels though they arise at
different times and from different parts of the medulla or pith? And, as
it exists previously to it, that the leaf is the parent of the bud?
This placentation of vegetable buds is clearly evinced from the
sweetness of the rising sap, and from its ceasing to rise as soon as the
leaves are expanded, and thus compleats the analogy between buds and
bulbs. Nor need we wonder at the length of the umbilical cords of buds
since that must correspond with their situation on the tree, in the same
manner as their lymphatics and arteries are proportionally elongated.
It does not appear probable that any umbilical artery attends these
placental absorbents, since, as there seems to be no system of veins in
vegetables to bring back the blood from the extremities of their
arteries, (except their pulmonary veins,) there could not be any
vegetable fluids to be returned to their placenta, which in vegetables
seems to be simply an organ for nutrition, whereas the placenta of the
animal foetus seems likewise to serve as a respiratory organ like the
gills of fishes.
NOTE XXXVI--VEGETABLE CIRCULATION.
_And refluent blood in milky eddies bends._
CANTO IV. l. 420.
The individuality of vegetable buds was spoken of before, and is
confirmed by the method of raising all kinds of trees by Mr. Barnes.
(Method of propagating Fruit Trees. 1759. Lond. Baldwin.) He cut a
branch into as many pieces as there were buds or leaves upon it, and
wiping the two wounded ends dry he quickly applied to each a cement,
previously warmed a little, which consisted principally of pitch, and
planted them in the earth. The use of this cement I suppose to consist
in its preventing the bud from bleeding to death, though the author
ascribes it to its antisceptic quality.
These buds of plants, which are thus each an individual vegetable, in
many circumstances resemble individual animals, but as animal bodies are
detached from the earth, and move from place to place in search of food,
and take that food at considerable intervals of time, and prepare it for
their nourishiment within their own bodies after it is taken, it is
evident they must require many organs and powers which are not necessary
to a stationary bud. As vegetables are immoveably fixed to the soil from
whence they draw their nourishment ready prepared, and this uniformly
not at returning intervals, it follows that in examining their anatome
we are not to look for muscles of locomotion, as arms and legs; nor for
organs to receive and prepare their nourishment, as a stomach and
bowels; nor for a reservoir for it after it is prepared, as a general
system of veins, which in locomotive animals contains and returns the
superfluous blood which is left after the various organs of secretion
have been supplied, by which contrivance they are enabled to live a long
time without new supplies of food.
The parts which we may expert to find in the anatome of vegetables
correspondent to those in the animal economy are, 1. A system of
absorbent vessels to imbibe the moisture of the earth similar to the
lacteal vessels, as in the roots of plants; and another system of
absorbents similar to the lymphatics of animal bodies, opening its
mouths on the internal cells and external surfaces of vegetables; and a
third system of absorbent vessels correspondent with those of the
placentation of the animal foetus. 2. A pulmonary system correspondent
to the lungs or gills of quadrupeds and fish, by which the fluid
absorbed by the lacteals and lymphatics may be exposed to the influence
of the air, this is done by the green leaves of plants, those in the air
resembling lungs, and those in the water resembling gills; and by the
petals of flowers. 3. Arterial systems to convey the fluid thus
elaborated to the various glands of the vegetable for the purposes of
its growth, nutrition, and various secretions. 4. The various glands
which separate from the vegetable blood the honey, wax, gum, resin,
starch, sugar, essential oil, &c. 5. The organs adapted for their
propagation or reproduction. 6. Muscles to perform several motions of
their parts.
I. The existence of that branch of the absorbent vessels of vegetables
which resembles the lacteals of animal bodies, and imbibes their
nutriment from the moist earth, is evinced by their growth so long as
moisture is applied to their roots, and their quickly withering when it
is withdrawn.
Besides these absorbents in the roots of plants there are others which
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