A History of Science, Volume 3
by
Henry Smith Williams
Part 4 out of 6
the popular excitement in England in 1716 over the
brilliant aurora of that year, which became famous
through Halley's description.
But after 1752, when Franklin dethroned the lightning,
all spectacular meteors came to be regarded as
natural phenomena, the aurora among the rest. Franklin
explained the aurora--which was seen commonly
enough in the eighteenth century, though only recorded
once in the seventeenth--as due to the accumulation of
electricity on the surface of polar snows, and its discharge
to the equator through the upper atmosphere.
Erasmus Darwin suggested that the luminosity might
be due to the ignition of hydrogen, which was supposed
by many philosophers to form the upper atmosphere.
Dalton, who first measured the height of the aurora,
estimating it at about one hundred miles, thought the
phenomenon due to magnetism acting on ferruginous
particles in the air, and his explanation was perhaps the
most popular one at the beginning of the last century.
Since then a multitude of observers have studied the
aurora, but the scientific grasp has found it as elusive in
fact as it seems to casual observation, and its exact
nature is as undetermined to-day as it was a hundred
years ago. There has been no dearth of theories concerning
it, however. Blot, who studied it in the Shetland
Islands in 1817, thought it due to electrified
ferruginous dust, the origin of which he ascribed to
Icelandic volcanoes. Much more recently the idea of
ferruginous particles has been revived, their presence
being ascribed not to volcanoes, but to the meteorites
constantly being dissipated in the upper atmosphere.
Ferruginous dust, presumably of such origin, has been
found on the polar snows, as well as on the snows of
mountain-tops, but whether it could produce the phenomena
of auroras is at least an open question.
Other theorists have explained the aurora as due to
the accumulation of electricity on clouds or on spicules
of ice in the upper air. Yet others think it due merely
to the passage of electricity through rarefied air itself.
Humboldt considered the matter settled in yet another
way when Faraday showed, in 1831, that magnetism
may produce luminous effects. But perhaps the prevailing
theory of to-day assumes that the aurora is due
to a current of electricity generated at the equator and
passing through upper regions of space, to enter the
earth at the magnetic poles--simply reversing the
course which Franklin assumed.
The similarity of the auroral light to that generated
in a vacuum bulb by the passage of electricity lends
support to the long-standing supposition that the aurora
is of electrical origin, but the subject still awaits
complete elucidation. For once even that mystery-
solver the spectroscope has been baffled, for the line it
sifts from the aurora is not matched by that of any
recognized substance. A like line is found in the
zodiacal light, it is true, but this is of little aid, for the
zodiacal light, though thought by some astronomers to
be due to meteor swarms about the sun, is held to be,
on the whole, as mysterious as the aurora itself.
Whatever the exact nature of the aurora, it has long
been known to be intimately associated with the phenomena
of terrestrial magnetism. Whenever a brilliant
aurora is visible, the world is sure to be visited
with what Humboldt called a magnetic storm--a
"storm" which manifests itself to human senses in no
way whatsoever except by deflecting the magnetic
needle and conjuring with the electric wire. Such
magnetic storms are curiously associated also with
spots on the sun--just how no one has explained,
though the fact itself is unquestioned. Sun-spots, too,
seem directly linked with auroras, each of these phenomena
passing through periods of greatest and least
frequency in corresponding cycles of about eleven
years' duration.
It was suspected a full century ago by Herschel that
the variations in the number of sun-spots had a direct
effect upon terrestrial weather, and he attempted to
demonstrate it by using the price of wheat as a criterion
of climatic conditions, meantime making careful observation
of the sun-spots. Nothing very definite came
of his efforts in this direction, the subject being far too
complex to be determined without long periods of observation.
Latterly, however, meteorologists, particularly
in the tropics, are disposed to think they find
evidence of some such connection between sun-spots
and the weather as Herschel suspected. Indeed, Mr.
Meldrum declares that there is a positive coincidence
between periods of numerous sun-spots and seasons
of excessive rain in India.
That some such connection does exist seems intrinsically
probable. But the modern meteorologist,
learning wisdom of the past, is extremely cautious
about ascribing casual effects to astronomical phenomena.
He finds it hard to forget that until recently all
manner of climatic conditions were associated with
phases of the moon; that not so very long ago showers
of falling-stars were considered "prognostic" of certain
kinds of weather; and that the "equinoctial storm"
had been accepted as a verity by every one, until
the unfeeling hand of statistics banished it from the
earth.
Yet, on the other hand, it is easily within the possibilities
that the science of the future may reveal associations
between the weather and sun-spots, auroras,
and terrestrial magnetism that as yet are hardly
dreamed of. Until such time, however, these phenomena
must feel themselves very grudgingly admitted
to the inner circle of meteorology. More and
more this science concerns itself, in our age of concentration
and specialization, with weather and climate.
Its votaries no longer concern themselves with stars or
planets or comets or shooting-stars--once thought the
very essence of guides to weather wisdom; and they are
even looking askance at the moon, and asking her to
show cause why she also should not be excluded from
their domain. Equally little do they care for the interior
of the earth, since they have learned that the
central emanations of heat which Mairan imagined as a
main source of aerial warmth can claim no such
distinction. Even such problems as why the magnetic
pole does not coincide with the geographical, and why
the force of terrestrial magnetism decreases from the
magnetic poles to the magnetic equator, as Humboldt
first discovered that it does, excite them only to
lukewarm interest; for magnetism, they say, is not
known to have any connection whatever with climate
or weather.
EVAPORATION, CLOUD FORMATION, AND DEW
There is at least one form of meteor, however, of
those that interested our forebears whose meteorological
importance they did not overestimate. This is the
vapor of water. How great was the interest in this
familiar meteor at the beginning of the century is attested
by the number of theories then extant regarding
it; and these conflicting theories bear witness also to
the difficulty with which the familiar phenomenon of
the evaporation of water was explained.
Franklin had suggested that air dissolves water much
as water dissolves salt, and this theory was still popular,
though Deluc had disproved it by showing that
water evaporates even more rapidly in a vacuum than
in air. Deluc's own theory, borrowed from earlier
chemists, was that evaporation is the chemical union
of particles of water with particles of the supposititious
element heat. Erasmus Darwin combined the
two theories, suggesting that the air might hold a
variable quantity of vapor in mere solution, and in
addition a permanent moiety in chemical combination
with caloric.
Undisturbed by these conflicting views, that strangely
original genius, John Dalton, afterwards to be known
as perhaps the greatest of theoretical chemists, took the
question in hand, and solved it by showing that water
exists in the air as an utterly independent gas. He
reached a partial insight into the matter in 1793, when
his first volume of meteorological essays was published;
but the full elucidation of the problem came to him in
1801. The merit of his studies was at once recognized,
but the tenability of his hypothesis was long and ardently
disputed.
While the nature of evaporation was in dispute, as a
matter of course the question of precipitation must be
equally undetermined. The most famous theory of the
period was that formulated by Dr. Hutton in a paper
read before the Royal Society of Edinburgh, and published
in the volume of transactions which contained
also the same author's epoch-making paper on geology.
This "theory of rain" explained precipitation as due to
the cooling of a current of saturated air by contact with
a colder current, the assumption being that the surplusage
of moisture was precipitated in a chemical
sense, just as the excess of salt dissolved in hot water is
precipitated when the water cools. The idea that the
cooling of the saturated air causes the precipitation of
its moisture is the germ of truth that renders this paper
of Hutton's important. All correct later theories build
on this foundation.
"Let us suppose the surface of this earth wholly
covered with water," said Hutton, "and that the sun
were stationary, being always vertical in one place;
then, from the laws of heat and rarefaction, there would
be formed a circulation in the atmosphere, flowing
from the dark and cold hemisphere to the heated and
illuminated place, in all directions, towards the place
of the greatest cold.
"As there is for the atmosphere of this earth a constant
cooling cause, this fluid body could only arrive
at a certain degree of heat; and this would be regularly
decreasing from the centre of illumination to the opposite
point of the globe, most distant from the light and
heat. Between these two regions of extreme heat and
cold there would, in every place, be found two streams
of air following in opposite directions. If those streams
of air, therefore, shall be supposed as both sufficiently
saturated with humidity, then, as they are of different
temperatures, there would be formed a continual condensation
of aqueous vapor, in some middle region of
the atmosphere, by the commixtion of part of those
two opposite streams.
"Hence there is reason to believe that in this supposed
case there would be formed upon the surface of
the globe three different regions--the torrid region, the
temperate, and the frigid. These three regions would
continue stationary; and the operations of each would
be continual. In the torrid region, nothing but evaporation
and heat would take place; no cloud could be
formed, because in changing the transparency of the
atmosphere to opacity it would be heated immediately
by the operation of light, and thus the condensed water
would be again evaporated. But this power of the
sun would have a termination; and it is these that
would begin the region of temperate heat and of continual
rain. It is not probable that the region of temperance
would reach far beyond the region of light; and
in the hemisphere of darkness there would be found a
region of extreme cold and perfect dryness.
"Let us now suppose the earth as turning on its axis
in the equinoctial situation. The torrid region would
thus be changed into a zone, in which there would be
night and day; consequently, here would be much
temperance, compared with the torrid region now
considered; and here perhaps there would be formed
periodical condensation and evaporation of humidity,
corresponding to the seasons of night and day. As temperance
would thus be introduced into the region of
torrid extremity, so would the effect of this change be
felt over all the globe, every part of which would now
be illuminated, consequently heated in some degree.
Thus we would have a line of great heat and evaporation,
graduating each way into a point of great cold
and congelation. Between these two extremes of heat
and cold there would be found in each hemisphere a
region of much temperance, in relation to heat, but of
much humidity in the atmosphere, perhaps of continual
rain and condensation.
"The supposition now formed must appear extremely
unfit for making this globe a habitable world in
every part; but having thus seen the effect of night
and day in temperating the effects of heat and cold in
every place, we are now prepared to contemplate the
effects of supposing this globe to revolve around the
sun with a certain inclination of its axis. By this
beautiful contrivance, that comparatively uninhabited
globe is now divided into two hemispheres, each of
which is thus provided with a summer and a winter
season. But our present view is limited to the
evaporation and condensation of humidity; and, in this
contrivance of the seasons, there must appear an ample
provision for those alternate operations in every part;
for as the place of the vertical sun is moved alternately
from one tropic to the other, heat and cold, the original
causes of evaporation and condensation, must be carried
over all the globe, producing either annual seasons
of rain or diurnal seasons of condensation and
evaporation, or both these seasons, more or less--that
is, in some degree.
"The original cause of motion in the atmosphere is
the influence of the sun heating the surface of the earth
exposed to that luminary. We have not supposed
that surface to have been of one uniform shape and
similar substance; from whence it has followed that
the annual propers of the sun, perhaps also the diurnal
propers, would produce a regular condensation of rain
in certain regions, and the evaporation of humidity in
others; and this would have a regular progress in certain
determined seasons, and would not vary. But
nothing can be more distant from this supposition, that
is the natural constitution of the earth; for the globe
is composed of sea and land, in no regular shape or
mixture, while the surface of the land is also irregular
with respect to its elevations and depressions, and
various with regard to the humidity and dryness of
that part which is exposed to heat as the cause of
evaporation. Hence a source of the most valuable
motions in the fluid atmosphere with aqueous vapor,
more or less, so far as other natural operations
will admit; and hence a source of the most irregular
commixture of the several parts of this elastic
fluid, whether saturated or not with aqueous vapor.
"According to the theory, nothing is required for the
production of rain besides the mixture of portions of
the atmosphere with humidity, and of mixing the
parts that are in different degrees of heat. But we
have seen the causes of saturating every portion of
the atmosphere with humidity and of mixing the
parts which are in different degrees of heat. Consequently,
over all the surface of the globe there should
happen occasionally rain and evaporation, more or
less; and also, in every place, those vicissitudes should
be observed to take place with some tendency to regularity,
which, however, may be so disturbed as to be
hardly distinguishable upon many occasions. Variable
winds and variable rains should be found in proportion
as each place is situated in an irregular mixture
of land and water; whereas regular winds should be
found in proportion to the uniformity of the surface;
and regular rains in proportion to the regular changes
of those winds by which the mixture of the atmosphere
necessary to the rain may be produced. But as it will
be acknowledged that this is the case in almost all this
earth where rain appears according to the conditions
here specified, the theory is found to be thus in conformity
with nature, and natural appearances are thus
explained by the theory."[1]
The next ambitious attempt to explain the phenomena
of aqueous meteors was made by Luke Howard, in
his remarkable paper on clouds, published in the
Philosophical Magazine in 1803--the paper in which
the names cirrus, cumulus, stratus, etc., afterwards so
universally adopted, were first proposed. In this paper
Howard acknowledges his indebtedness to Dalton for
the theory of evaporation; yet he still clings to the idea
that the vapor, though independent of the air, is combined
with particles of caloric. He holds that clouds
are composed of vapor that has previously risen from
the earth, combating the opinions of those who believe
that they are formed by the union of hydrogen and
oxygen existing independently in the air; though he
agrees with these theorists that electricity has entered
largely into the modus operandi of cloud formation. He
opposes the opinion of Deluc and De Saussure that
clouds are composed of particles of water in the form
of hollow vesicles (miniature balloons, in short, perhaps
filled with hydrogen), which untenable opinion
was a revival of the theory as to the formation of all
vapor which Dr. Halley had advocated early in the
eighteenth century.
Of particular interest are Howard's views as to the
formation of dew, which he explains as caused by the
particles of caloric forsaking the vapor to enter the cool
body, leaving the water on the surface. This comes as
near the truth, perhaps, as could be expected while the
old idea as to the materiality of heat held sway. Howard
believed, however, that dew is usually formed in
the air at some height, and that it settles to the surface,
opposing the opinion, which had gained vogue in France
and in America (where Noah Webster prominently advocated
it), that dew ascends from the earth.
The complete solution of the problem of dew formation--
which really involved also the entire question of
precipitation of watery vapor in any form--was made
by Dr. W. C. Wells, a man of American birth, whose
life, however, after boyhood, was spent in Scotland
(where as a young man he enjoyed the friendship of
David Hume) and in London. Inspired, no doubt,
by the researches of Mack, Hutton, and their confreres
of that Edinburgh school, Wells made observations on
evaporation and precipitation as early as 1784, but
other things claimed his attention; and though he asserts
that the subject was often in his mind, he did not
take it up again in earnest until about 1812.
Meantime the observations on heat of Rumford and
Davy and Leslie had cleared the way for a proper
interpretation of the facts--about the facts themselves
there had long been practical unanimity of opinion.
Dr. Black, with his latent-heat observations, had really
given the clew to all subsequent discussions of the
subject of precipitation of vapor; and from this time on
it had been known that heat is taken up when water
evaporates, and given out again when it condenses.
Dr. Darwin had shown in 1788, in a paper before the
Royal Society, that air gives off heat on contracting
and takes it up on expanding; and Dalton, in his
essay of 1793, had explained this phenomenon as due
to the condensation and vaporization of the water contained
in the air.
But some curious and puzzling observations which
Professor Patrick Wilson, professor of astronomy in
the University of Glasgow, had communicated to the
Royal Society of Edinburgh in 1784, and some similar
ones made by Mr. Six, of Canterbury, a few years later,
had remained unexplained. Both these gentlemen
observed that the air is cooler where dew is forming than
the air a few feet higher, and they inferred that the dew
in forming had taken up heat, in apparent violation of
established physical principles.
It remained for Wells, in his memorable paper of
1816, to show that these observers had simply placed
the cart before the horse. He made it clear that the
air is not cooler because the dew is formed, but that the
dew is formed because the air is cooler--having become
so through radiation of heat from the solids on which
the dew forms. The dew itself, in forming, gives out
its latent heat, and so tends to equalize the temperature.
Wells's paper is so admirable an illustration of the
lucid presentation of clearly conceived experiments
and logical conclusions that we should do it injustice
not to present it entire. The author's mention of the
observations of Six and Wilson gives added value to his
own presentation.
Dr. Wells's Essay on Dew
"I was led in the autumn of 1784, by the event of a
rude experiment, to think it probable that the formation
of dew is attended with the production of cold.
In 1788, a paper on hoar-frost, by Mr. Patrick Wilson,
of Glasgow, was published in the first volume of the
Transactions of the Royal Society of Edinburgh, by
which it appeared that this opinion bad been entertained
by that gentleman before it had occurred to
myself. In the course of the same year, Mr. Six, of
Canterbury, mentioned in a paper communicated to
the Royal Society that on clear and dewy nights he
always found the mercury lower in a thermometer laid
upon the ground in a meadow in his neighborhood than
it was in a similar thermometer suspended in the air six
feet above the former; and that upon one night the
difference amounted to five degrees of Fahrenheit's
scale. Mr. Six, however, did not suppose, agreeably to
the opinion of Mr. Wilson and myself, that the cold was
occasioned by the formation of dew, but imagined that
it proceeded partly from the low temperature of the
air, through which the dew, already formed in the
atmosphere, had descended, and partly from the
evaporation of moisture from the ground, on which his
thermometer had been placed. The conjecture of Mr.
Wilson and the observations of Mr. Six, together with
many facts which I afterwards learned in the course
of reading, strengthened my opinion; but I made no
attempt, before the autumn of 1811, to ascertain by
experiment if it were just, though it had in the mean
time almost daily occurred to my thoughts. Happening,
in that season, to be in that country in a clear and
calm night, I laid a thermometer upon grass wet with
dew, and suspended a second in the air, two feet above
the other. An hour afterwards the thermometer on
the grass was found to be eight degrees lower, by
Fahrenheit's division, than the one in the air. Similar
results having been obtained from several similar
experiments, made during the same autumn, I determined
in the next spring to prosecute the subject with
some degree of steadiness, and with that view went
frequently to the house of one of my friends who lives
in Surrey.
At the end of two months I fancied that I had
collected information worthy of being published; but,
fortunately, while preparing an account of it I met by
accident with a small posthumous work by Mr. Six,
printed at Canterbury in 1794, in which are related
differences observed on dewy nights between thermometers
placed upon grass and others in the air that
are much greater than those mentioned in the paper
presented by him to the Royal Society in 1788. In this
work, too, the cold of the grass is attributed, in agreement
with the opinion of Mr. Wilson, altogether to the
dew deposited upon it. The value of my own observations
appearing to me now much diminished, though
they embraced many points left untouched by Mr. Six,
I gave up my intentions of making them known. Shortly
after, however, upon considering the subject more
closely, I began to suspect that Mr. Wilson, Mr. Six,
and myself had all committed an error regarding the
cold which accompanies dew as an effect of the formation
of that fluid. I therefore resumed my experiments,
and having by means of them, I think, not only
established the justness of my suspicions, but ascertained
the real cause both of dew and of several other
natural appearances which have hitherto received no
sufficient explanation, I venture now to submit to the
consideration of the learned an account of some of
my labors, without regard to the order of time in
which they were performed, and of various conclusions
which may be drawn from them, mixed with facts and
opinions already published by others:
"There are various occurrences in nature which
seem to me strictly allied to dew, though their relation
to it be not always at first sight perceivable. The
statement and explanation of several of these will form
the concluding part of the present essay.
"1. I observed one morning, in winter, that the insides
of the panes of glass in the windows of my bedchamber
were all of them moist, but that those which
had been covered by an inside shutter during the night
were much more so than the others which had been
uncovered. Supposing that this diversity of appearance
depended upon a difference of temperature, I
applied the naked bulbs of two delicate thermometers
to a covered and uncovered pane; on which I found
that the former was three degrees colder than the
latter. The air of the chamber, though no fire was
kept in it, was at this time eleven and one-half degrees
warmer than that without. Similar experiments
were made on many other mornings, the results of
which were that the warmth of the internal air exceeded
that of the external from eight to eighteen degrees,
the temperature of the covered panes would be
from one to five degrees less than the uncovered; that
the covered were sometimes dewed, while the uncovered
were dry; that at other times both were free from
moisture; that the outsides of the covered and uncovered
panes had similar differences with respect to heat,
though not so great as those of the inner surfaces; and
that no variation in the quantity of these differences
was occasioned by the weather's being cloudy or fair,
provided the heat of the internal air exceeded that of
the external equally in both of those states of the
atmosphere.
"The remote reason of these differences did not immediately
present itself. I soon, however, saw that
the closed shutter shielded the glass which it covered
from the heat that was radiated to the windows by
the walls and furniture of the room, and thus kept it
nearer to the temperature of the external air than
those parts could be which, from being uncovered, received
the heat emitted to them by the bodies just
mentioned.
"In making these experiments, I seldom observed
the inside of any pane to be more than a little damped,
though it might be from eight to twelve degrees colder
than the general mass of the air in the room; while, in
the open air, I had often found a great dew to form on
substances only three or four degrees colder than the
atmosphere. This at first surprised me; but the cause
now seems plain. The air of the chamber had once
been a portion of the external atmosphere, and had
afterwards been heated, when it could receive little accessories
to its original moisture. It constantly required
being cooled considerably before it was even
brought back to its former nearness to repletion with
water; whereas the whole external air is commonly, at
night, nearly replete with moisture, and therefore
readily precipitates dew on bodies only a little colder
than itself.
"When the air of a room is warmer than the external
atmosphere, the effect of an outside shutter on the
temperature of the glass of the window will be directly
opposite to what has just been stated; since it must
prevent the radiation, into the atmosphere, of the heat
of the chamber transmitted through the glass.
"2. Count Rumford appears to have rightly conjectured
that the inhabitants of certain hot countries,
who sleep at nights on the tops of their houses, are
cooled during this exposure by the radiation of their
heat to the sky; or, according to his manner of expression,
by receiving frigorific rays from the heavens.
Another fact of this kind seems to be the greater chill
which we often experience upon passing at night from
the cover of a house into the air than might have been
expected from the cold of the external atmosphere.
The cause, indeed, is said to be the quickness of transition
from one situation to another. But if this were
the whole reason, an equal chill would be felt in the day,
when the difference, in point of heat, between the internal
and external air was the same as at night, which
is not the case. Besides, if I can trust my own observation,
the feeling of cold from this cause is more remarkable
in a clear than in a cloudy night, and in the
country than in towns. The following appears to be
the manner in which these things are chiefly to be explained:
"During the day our bodies while in the open air,
although not immediately exposed to the sun's rays, are
yet constantly deriving heat from them by means of
the reflection of the atmosphere. This heat, though it
produces little change on the temperature of the air
which it traverses, affords us some compensation for
the heat which we radiate to the heavens. At night,
also, if the sky be overcast, some compensation will be
made to us, both in the town and in the country,
though in a less degree than during the day, as the
clouds will remit towards the earth no inconsiderable
quantity of heat. But on a clear night, in an open part
of the country, nothing almost can be returned to us
from above in place of the heat which we radiate upward.
In towns, however, some compensation will be
afforded even on the clearest nights for the heat
which we lose in the open air by that which is radiated
to us from the sun round buildings.
To our loss of heat by radiation at times that we
derive little compensation from the radiation of other
bodies is probably to be attributed a great part of the
hurtful effects of the night air. Descartes says that
these are not owing to dew, as was the common opinion
of his contemporaries, but to the descent of certain
noxious vapors which have been exhaled from the earth
during the heat of the day, and are afterwards condensed
by the cold of a serene night. The effects in
question certainly cannot be occasioned by dew, since
that fluid does not form upon a healthy human body
in temperate climates; but they may, notwithstanding,
arise from the same cause that produces dew on those
substances which do not, like the human body, possess
the power of generating heat for the supply of what
they lose by radiation or any other means."[2]
This explanation made it plain why dew forms on a
clear night, when there are no clouds to reflect the radiant
heat. Combined with Dalton's theory that vapor
is an independent gas, limited in quantity in any given
space by the temperature of that space, it solved the
problem of the formation of clouds, rain, snow, and
hoar-frost. Thus this paper of Wells's closed the epoch
of speculation regarding this field of meteorology, as
Hutton's paper of 1784 had opened it. The fact that
the volume containing Hutton's paper contained also
his epoch-making paper on geology finds curiously a
duplication in the fact that Wells's volume contained
also his essay on Albinism, in which the doctrine of
natural selection was for the first time formulated, as
Charles Darwin freely admitted after his own efforts
had made the doctrine famous.
ISOTHERMS AND OCEAN CURRENTS
The very next year after Dr. Wells's paper was published
there appeared in France the third volume of
the Memoires de Physique et de Chimie de la Societe
d'Arcueil, and a new epoch in meteorology was inaugurated.
The society in question was numerically an inconsequential
band, listing only a dozen members; but every name was a famous
one: Arago, Berard, Berthollet, Biot, Chaptal, De Candolle,
Dulong, Gay-Lussac, Humboldt, Laplace, Poisson, and Thenard--rare
spirits every one. Little danger that the memoirs of such a band
would be relegated to the dusty shelves where most proceedings of
societies belong--no milk-for-babes fare would be served to such
a company.
The particular paper which here interests us closes
this third and last volume of memoirs. It is entitled
"Des Lignes Isothermes et de la Distribution de la
Chaleursurle Globe." The author is Alexander Humboldt.
Needless to say, the topic is handled in a masterly
manner. The distribution of heat on the surface of the
globe, on the mountain-sides, in the interior of the
earth; the causes that regulate such distribution; the
climatic results--these are the topics discussed. But
what gives epochal character to the paper is the introduction
of those isothermal lines circling the earth in
irregular course, joining together places having the
same mean annual temperature, and thus laying the
foundation for a science of comparative climatology.
It is true the attempt to study climates comparatively
was not new. Mairan had attempted it in those
papers in which he developed his bizarre ideas as to
central emanations of heat. Euler had brought his
profound mathematical genius to bear on the topic,
evolving the "extraordinary conclusion that under the
equator at midnight the cold ought to be more rigorous
than at the poles in winter." And in particular Richard
Kirwan, the English chemist, had combined the
mathematical and the empirical methods and calculated
temperatures for all latitudes. But Humboldt
differs from all these predecessors in that he grasps the
idea that the basis of all such computations should be
not theory, but fact. He drew his isothermal lines not
where some occult calculation would locate them on an
ideal globe, but where practical tests with the thermometer
locate them on our globe as it is. London,
for example, lies in the same latitude as the southern
extremity of Hudson Bay; but the isotherm of London,
as Humboldt outlines it, passes through Cincinnati.
Of course such deviations of climatic conditions between
places in the same latitude had long been known.
As Humboldt himself observes, the earliest settlers of
America were astonished to find themselves subjected
to rigors of climate for which their European experience
had not at all prepared them. Moreover, sagacious
travellers, in particular Cook's companion on his second
voyage, young George Forster, had noted as a general
principle that the western borders of continents in
temperate regions are always warmer than corresponding
latitudes of their eastern borders; and of course the
general truth of temperatures being milder in the vicinity
of the sea than in the interior of continents had
long been familiar. But Humboldt's isothermal lines
for the first time gave tangibility to these ideas, and
made practicable a truly scientific study of comparative
climatology.
In studying these lines, particularly as elaborated by
further observations, it became clear that they are by
no means haphazard in arrangement, but are dependent
upon geographical conditions which in most cases
are not difficult to determine. Humboldt himself
pointed out very clearly the main causes that tend to
produce deviations from the average--or, as Dove
later on called it, the normal--temperature of any given
latitude. For example, the mean annual temperature
of a region (referring mainly to the northern hemisphere)
is raised by the proximity of a western coast;
by a divided configuration of the continent into peninsulas;
by the existence of open seas to the north or of
radiating continental surfaces to the south; by mountain
ranges to shield from cold winds; by the infrequency
of swamps to become congealed; by the absence
of woods in a dry, sandy soil; and by the serenity
of sky in the summer months and the vicinity of an
ocean current bringing water which is of a higher
temperature than that of the surrounding sea.
Conditions opposite to these tend, of course,
correspondingly to lower the temperature. In a word,
Humboldt says the climatic distribution of heat depends
on the relative distribution of land and sea, and
on the "hypsometrical configuration of the continents";
and he urges that "great meteorological phenomena
cannot be comprehended when considered independently
of geognostic relations"--a truth which,
like most other general principles, seems simple enough
once it is pointed out.
With that broad sweep of imagination which characterized
him, Humboldt speaks of the atmosphere as the
"aerial ocean, in the lower strata and on the shoals of
which we live," and he studies the atmospheric phenomena
always in relation to those of that other ocean
of water. In each of these oceans there are vast permanent
currents, flowing always in determinate directions,
which enormously modify the climatic conditions
of every zone. The ocean of air is a vast maelstrom,
boiling up always under the influence of the sun's heat
at the equator, and flowing as an upper current towards
either pole, while an undercurrent from the poles,
which becomes the trade-winds, flows towards the
equator to supply its place.
But the superheated equatorial air, becoming chilled,
descends to the surface in temperate latitudes, and continues
its poleward journey as the anti-trade-winds.
The trade-winds are deflected towards the west, because
in approaching the equator they constantly pass
over surfaces of the earth having a greater and greater
velocity of rotation, and so, as it were, tend to lag behind--
an explanation which Hadley pointed out in
1735, but which was not accepted until Dalton independently
worked it out and promulgated it in 1793.
For the opposite reason, the anti-trades are deflected
towards the east; hence it is that the western, borders
of continents in temperate zones are bathed in moist
sea-breezes, while their eastern borders lack this cold-
dispelling influence.
In the ocean of water the main currents run as more
sharply circumscribed streams--veritable rivers in the
sea. Of these the best known and most sharply circumscribed
is the familiar Gulf Stream, which has its
origin in an equatorial current, impelled westward by
trade-winds, which is deflected northward in the main
at Cape St. Roque, entering the Caribbean Sea and Gulf
of Mexico, to emerge finally through the Strait of
Florida, and journey off across the Atlantic to warm
the shores of Europe.
Such, at least, is the Gulf Stream as Humboldt understood
it. Since his time, however, ocean currents in
general, and this one in particular, have been the subject
of no end of controversy, it being hotly disputed
whether either causes or effects of the Gulf Stream are
just what Humboldt, in common with others of his
time, conceived them to be. About the middle of the
century Lieutenant M. F. Maury, the distinguished
American hydrographer and meteorologist, advocated
a theory of gravitation as the chief cause of the currents,
claiming that difference in density, due to difference
in temperature and saltness, would sufficiently
account for the oceanic circulation. This theory
gained great popularity through the wide circulation
of Maury's Physical Geography of the Sea, which is said
to have passed through more editions than any other
scientific book of the period; but it was ably and
vigorously combated by Dr. James Croll, the Scottish
geologist, in his Climate and Time, and latterly the old
theory that ocean currents are due to the trade-winds
has again come into favor. Indeed, very recently a
model has been constructed, with the aid of which it is
said to have been demonstrated that prevailing winds
in the direction of the actual trade-winds would produce
such a current as the Gulf Stream.
Meantime, however, it is by no means sure that
gravitation does not enter into the case to the extent
of producing an insensible general oceanic circulation,
independent of the Gulf Stream and similar marked
currents, and similar in its larger outlines to the polar-
equatorial circulation of the air. The idea of such
oceanic circulation was first suggested in detail by
Professor Lenz, of St. Petersburg, in 1845, but it
was not generally recognized until Dr. Carpenter
independently hit upon the idea more than twenty
years later. The plausibility of the conception is obvious;
yet the alleged fact of such circulation has
been hotly disputed, and the question is still sub
judice.
But whether or not such general circulation of ocean
water takes place, it is beyond dispute that the recognized
currents carry an enormous quantity of heat
from the tropics towards the poles. Dr. Croll, who has
perhaps given more attention to the physics of the
subject than almost any other person, computes that
the Gulf Stream conveys to the North Atlantic one-
fourth as much heat as that body receives directly from
the sun, and he argues that were it not for the transportation
of heat by this and similar Pacific currents,
only a narrow tropical region of the globe would be
warm enough for habitation by the existing faunas.
Dr. Croll argues that a slight change in the relative
values of northern and southern trade-winds (such as
he believes has taken place at various periods in the
past) would suffice to so alter the equatorial current
which now feeds the Gulf Stream that its main bulk
would be deflected southward instead of northward,
by the angle of Cape St. Roque. Thus the Gulf Stream
would be nipped in the bud, and, according to Dr.
Croll's estimates, the results would be disastrous for the
northern hemisphere. The anti-trades, which now are
warmed by the Gulf Stream, would then blow as cold
winds across the shores of western Europe, and in all
probability a glacial epoch would supervene throughout
the northern hemisphere.
The same consequences, so far as Europe is concerned
at least, would apparently ensue were the Isthmus
of Panama to settle into the sea, allowing the
Caribbean current to pass into the Pacific. But the
geologist tells us that this isthmus rose at a comparatively
recent geological period, though it is hinted that
there had been some time previously a temporary land
connection between the two continents. Are we to
infer, then, that the two Americas in their unions and
disunions have juggled with the climate of the other
hemisphere? Apparently so, if the estimates made of
the influence of the Gulf Stream be tenable. It is a
far cry from Panama to Russia. Yet it seems within
the possibilities that the meteorologist may learn from
the geologist of Central America something that will
enable him to explain to the paleontologist of Europe
how it chanced that at one time the mammoth and
rhinoceros roamed across northern Siberia, while at
another time the reindeer and musk-ox browsed along
the shores of the Mediterranean.
Possibilities, I said, not probabilities. Yet even the
faint glimmer of so alluring a possibility brings home to
one with vividness the truth of Humboldt's perspicuous
observation that meteorology can be properly comprehended
only when studied in connection with the
companion sciences. There are no isolated phenomena
in nature.
CYCLONES AND ANTI-CYCLONES
Yet, after all, it is not to be denied that the chief
concern of the meteorologist must be with that other
medium, the "ocean of air, on the shoals of which we
live." For whatever may be accomplished by water
currents in the way of conveying heat, it is the wind
currents that effect the final distribution of that heat.
As Dr. Croll has urged, the waters of the Gulf Stream
do not warm the shores of Europe by direct contact,
but by warming the anti-trade-winds, which subsequently
blow across the continent. And everywhere
the heat accumulated by water becomes effectual in
modifying climate, not so much by direct radiation as
by diffusion through the medium of the air.
This very obvious importance of aerial currents led
to their practical study long before meteorology had
any title to the rank of science, and Dalton's explanation
of the trade-winds had laid the foundation for a
science of wind dynamics before the beginning of the
nineteenth century. But no substantial further advance
in this direction was effected until about 1827,
when Heinrich W. Dove, of Konigsberg, afterwards to
be known as perhaps the foremost meteorologist of his
generation, included the winds among the subjects of
his elaborate statistical studies in climatology.
Dove classified the winds as permanent, periodical,
and variable. His great discovery was that all winds,
of whatever character, and not merely the permanent
winds, come under the influence of the earth's rotation
in such a way as to be deflected from their course, and
hence to take on a gyratory motion--that, in short, all
local winds are minor eddies in the great polar-equatorial
whirl, and tend to reproduce in miniature the character
of that vast maelstrom. For the first time, then,
temporary or variable winds were seen to lie within the
province of law.
A generation later, Professor William Ferrel, the
American meteorologist, who had been led to take up
the subject by a perusal of Maury's discourse on ocean
winds, formulated a general mathematical law, to the
effect that any body moving in a right line along the
surface of the earth in any direction tends to have its
course deflected, owing to the earth's rotation, to the
right hand in the northern and to the left hand in
the southern hemisphere. This law had indeed been
stated as early as 1835 by the French physicist Poisson,
but no one then thought of it as other than a mathematical
curiosity; its true significance was only understood
after Professor Ferrel had independently rediscovered
it (just as Dalton rediscovered Hadley's forgotten
law of the trade-winds) and applied it to the
motion of wind currents.
Then it became clear that here is a key to the phenomena
of atmospheric circulation, from the great
polar-equatorial maelstrom which manifests itself in
the trade-winds to the most circumscribed riffle which
is announced as a local storm. And the more the phenomena
were studied, the more striking seemed the
parallel between the greater maelstrom and these lesser
eddies. Just as the entire atmospheric mass of each
hemisphere is seen, when viewed as a whole, to be carried
in a great whirl about the pole of that hemisphere,
so the local disturbances within this great tide are
found always to take the form of whirls about a local
storm-centre--which storm-centre, meantime, is carried
along in the major current, as one often sees a
little whirlpool in the water swept along with the main
current of the stream. Sometimes, indeed, the local
eddy, caught as it were in an ancillary current of the
great polar stream, is deflected from its normal course
and may seem to travel against the stream; but such
deviations are departures from the rule. In the great
majority of cases, for example, in the north temperate
zone, a storm-centre (with its attendant local whirl)
travels to the northeast, along the main current of the
anti-trade-wind, of which it is a part; and though
exceptionally its course may be to the southeast instead,
it almost never departs so widely from the main channel
as to progress to the westward. Thus it is that
storms sweeping over the United States can be announced,
as a rule, at the seaboard in advance of their
coming by telegraphic communication from the interior,
while similar storms come to Europe off the
ocean unannounced. Hence the more practical availability
of the forecasts of weather bureaus in the former
country.
But these local whirls, it must be understood, are
local only in a very general sense of the word, inasmuch
as a single one may be more than a thousand miles in
diameter, and a small one is two or three hundred miles
across. But quite without regard to the size of the
whirl, the air composing it conducts itself always in one
of two ways. It never whirls in concentric circles; it
always either rushes in towards the centre in a descending
spiral, in which case it is called a cyclone, or it
spreads out from the centre in a widening spiral, in
which case it is called an anti-cyclone. The word
cyclone is associated in popular phraseology with a
terrific storm, but it has no such restriction in technical
usage. A gentle zephyr flowing towards a "storm-
centre" is just as much a cyclone to the meteorologist
as is the whirl constituting a West-Indian hurricane.
Indeed, it is not properly the wind itself that is called
the cyclone in either case, but the entire system of
whirls--including the storm-centre itself, where there
may be no wind at all.
What, then, is this storm-centre? Merely an area
of low barometric pressure--an area where the air has
become lighter than the air of surrounding regions.
Under influence of gravitation the air seeks its level
just as water does; so the heavy air comes flowing in
from all sides towards the low-pressure area, which thus
becomes a "storm-centre." But the inrushing currents
never come straight to their mark. In accordance with
Ferrel's law, they are deflected to the right, and the
result, as will readily be seen, must be a vortex current,
which whirls always in one direction--namely, from
left to right, or in the direction opposite to that of the
hands of a watch held with its face upward. The
velocity of the cyclonic currents will depend largely
upon the difference in barometric pressure between the
storm-centre and the confines of the cyclone system.
And the velocity of the currents will determine to some
extent the degree of deflection, and hence the exact
path of the descending spiral in which the wind approaches
the centre. But in every case and in every
part of the cyclone system it is true, as Buys Ballot's
famous rule first pointed out, that a person standing
with his back to the wind has the storm-centre at his
left.
The primary cause of the low barometric pressure
which marks the storm-centre and establishes the cyclone
is expansion of the air through excess of temperature.
The heated air, rising into cold upper regions,
has a portion of its vapor condensed into clouds,
and now a new dynamic factor is added, for each particle
of vapor, in condensing, gives up its modicum of
latent heat. Each pound of vapor thus liberates, according
to Professor Tyndall's estimate, enough heat
to melt five pounds of cast iron; so the amount given
out where large masses of cloud are forming must enormously
add to the convection currents of the air, and
hence to the storm-developing power of the forming
cyclone. Indeed, one school of meteorologists, of
whom Professor Espy was the leader, has held that,
without such added increment of energy constantly
augmenting the dynamic effects, no storm could long
continue in violent action. And it is doubted whether
any storm could ever attain, much less continue, the
terrific force of that most dreaded of winds of temperate
zones, the tornado--a storm which obeys all the laws
of cyclones, but differs from ordinary cyclones in having
a vortex core only a few feet or yards in diameter--
without the aid of those great masses of condensing
vapor which always accompany it in the form of storm-
clouds.
The anti-cyclone simply reverses the conditions of
the cyclone. Its centre is an area of high pressure,
and the air rushes out from it in all directions towards
surrounding regions of low pressure. As before, all
parts of the current will be deflected towards the right,
and the result, clearly, is a whirl opposite in direction
to that of the cyclone. But here there is a tendency
to dissipation rather than to concentration of energy,
hence, considered as a storm-generator, the anti-
cyclone is of relative insignificance.
In particular the professional meteorologist who
conducts a "weather bureau"--as, for example, the
chief of the United States signal-service station in
New York--is so preoccupied with the observation of
this phenomenon that cyclone-hunting might be said
to be his chief pursuit. It is for this purpose, in the
main, that government weather bureaus or signal-
service departments have been established all over the
world. Their chief work is to follow up cyclones, with
the aid of telegraphic reports, mapping their course
and recording the attendant meteorological conditions.
Their so-called predictions or forecasts are essentially
predications, gaining locally the effect of predictions
because the telegraph outstrips the wind.
At only one place on the globe has it been possible
as yet for the meteorologist to make long-time
forecasts meriting the title of predictions. This is in the
middle Ganges Valley of northern India. In this country
the climatic conditions are largely dependent upon
the periodical winds called monsoons, which blow
steadily landward from April to October, and seaward
from October to April. The summer monsoons bring
the all-essential rains; if they are delayed or restricted
in extent, there will be drought and consequent famine.
And such restriction of the monsoon is likely to result
when there has been an unusually deep or very late
snowfall on the Himalayas, because of the lowering of
spring temperature by the melting snow. Thus here
it is possible, by observing the snowfall in the mountains,
to predict with some measure of success the average
rainfall of the following summer. The drought of
1896, with the consequent famine and plague that devastated
India the following winter, was thus predicted
some months in advance.
This is the greatest present triumph of practical meteorology.
Nothing like it is yet possible anywhere in
temperate zones. But no one can say what may not
be possible in times to come, when the data now being
gathered all over the world shall at last be co-ordinated,
classified, and made the basis of broad inductions.
Meteorology is pre-eminently a science of the future.
VI
MODERN THEORIES OF HEAT AND LIGHT
THE eighteenth-century philosopher made great
strides in his studies of the physical properties of
matter and the application of these properties in
mechanics, as the steam-engine, the balloon, the optic
telegraph, the spinning-jenny, the cotton-gin, the
chronometer, the perfected compass, the Leyden jar,
the lightning-rod, and a host of minor inventions testify.
In a speculative way he had thought out more or
less tenable conceptions as to the ultimate nature of
matter, as witness the theories of Leibnitz and Boscovich
and Davy, to which we may recur. But he had
not as yet conceived the notion of a distinction between
matter and energy, which is so fundamental to the
physics of a later epoch. He did not speak of heat,
light, electricity, as forms of energy or "force"; he conceived
them as subtile forms of matter--as highly attenuated
yet tangible fluids, subject to gravitation and
chemical attraction; though he had learned to measure
none of them but heat with accuracy, and this one he
could test only within narrow limits until late in the
century, when Josiah Wedgwood, the famous potter,
taught him to gauge the highest temperatures with the
clay pyrometer.
He spoke of the matter of heat as being the most universally
distributed fluid in nature; as entering in some
degree into the composition of nearly all other substances;
as being sometimes liquid, sometimes condensed
or solid, and as having weight that could be detected
with the balance. Following Newton, he spoke
of light as a "corpuscular emanation" or fluid, composed
of shining particles which possibly are transmutable
into particles of heat, and which enter into chemical
combination with the particles of other forms of
matter. Electricity he considered a still more subtile
kind of matter-perhaps an attenuated form of
light. Magnetism, "vital fluid," and by some even
a "gravic fluid," and a fluid of sound were placed
in the same scale; and, taken together, all these supposed
subtile forms of matter were classed as "imponderables."
This view of the nature of the "imponderables" was
in some measure a retrogression, for many seventeenth-
century philosophers, notably Hooke and Huygens and
Boyle, had held more correct views; but the materialistic
conception accorded so well with the eighteenth-
century tendencies of thought that only here and there
a philosopher like Euler called it in question, until well
on towards the close of the century. Current speech
referred to the materiality of the "imponderables "
unquestioningly. Students of meteorology--a science
that was just dawning--explained atmospheric phenomena
on the supposition that heat, the heaviest
imponderable, predominated in the lower atmosphere,
and that light, electricity, and magnetism prevailed in
successively higher strata. And Lavoisier, the most
philosophical chemist of the century, retained heat and
light on a par with oxygen, hydrogen, iron, and the
rest, in his list of elementary substances.
COUNT RUMFORD AND THE VIBRATORY THEORY OF HEAT
But just at the close of the century the confidence in
the status of the imponderables was rudely shaken in
the minds of philosophers by the revival of the old idea
of Fra Paolo and Bacon and Boyle, that heat, at any
rate, is not a material fluid, but merely a mode of motion
or vibration among the particles of "ponderable"
matter. The new champion of the old doctrine as to
the nature of heat was a very distinguished philosopher
and diplomatist of the time, who, it may be worth recalling,
was an American. He was a sadly expatriated
American, it is true, as his name, given all the official
appendages, will amply testify; but he had been born
and reared in a Massachusetts village none the less, and
he seems always to have retained a kindly interest in
the land of his nativity, even though he lived abroad in
the service of other powers during all the later years of
his life, and was knighted by England, ennobled by
Bavaria, and honored by the most distinguished scientific
bodies of Europe. The American, then, who
championed the vibratory theory of heat, in opposition
to all current opinion, in this closing era of the eighteenth
century, was Lieutenant-General Sir Benjamin
Thompson, Count Rumford, F.R.S.
Rumford showed that heat may be produced in indefinite
quantities by friction of bodies that do not
themselves lose any appreciable matter in the process,
and claimed that this proves the immateriality of heat.
Later on he added force to the argument by proving,
in refutation of the experiments of Bowditch, that no
body either gains or loses weight in virtue of being
heated or cooled. He thought he had proved that heat
is only a form of motion.
His experiment for producing indefinite quantities
of heat by friction is recorded by him in his paper entitled,
"Inquiry Concerning the Source of Heat Excited
by Friction."
"Being engaged, lately, in superintending the boring
of cannon in the workshops of the military arsenal
at Munich," he says, "I was struck with the very
considerable degree of heat which a brass gun acquires
in a short time in being bored; and with the still more
intense heat (much greater than that of boiling water,
as I found by experiment) of the metallic chips separated
from it by the borer.
"Taking a cannon (a brass six-pounder), cast solid,
and rough, as it came from the foundry, and fixing it
horizontally in a machine used for boring, and at the
same time finishing the outside of the cannon by turning,
I caused its extremity to be cut off; and by turning
down the metal in that part, a solid cylinder was
formed, 7 3/4 inches in diameter and 9 8/10 inches long;
which, when finished, remained joined to the rest of the
metal (that which, properly speaking, constituted the
cannon) by a small cylindrical neck, only 2 1/5 inches
in diameter and 3 8/10 inches long.
"This short cylinder, which was supported in its
horizontal position, and turned round its axis by
means of the neck by which it remained united to the
cannon, was now bored with the horizontal borer used
in boring cannon.
"This cylinder being designed for the express purpose
of generating heat by friction, by having a blunt
borer forced against its solid bottom at the same time
that it should be turned round its axis by the force of
horses, in order that the heat accumulated in the cylinder
might from time to time be measured, a small,
round hole 0.37 of an inch only in diameter and 4.2
inches in depth, for the purpose of introducing a small
cylindrical mercurial thermometer, was made in it, on
one side, in a direction perpendicular to the axis of the
cylinder, and ending in the middle of the solid part of
the metal which formed the bottom of the bore.
"At the beginning of the experiment, the temperature
of the air in the shade, as also in the cylinder, was
just sixty degrees Fahrenheit. At the end of thirty
minutes, when the cylinder had made 960 revolutions
about its axis, the horses being stopped, a cylindrical
mercury thermometer, whose bulb was 32/100 of an inch
in diameter and 3 1/4 inches in length, was introduced
into the hole made to receive it in the side of the cylinder,
when the mercury rose almost instantly to one
hundred and thirty degrees.
"In order, by one decisive experiment, to determine
whether the air of the atmosphere had any part or not
in the generation of the heat, I contrived to repeat the
experiment under circumstances in which it was evidently
impossible for it to produce any effect whatever.
By means of a piston exactly fitted to the mouth of the
bore of the cylinder, through the middle of which piston
the square iron bar, to the end of which the blunt
steel borer was fixed, passed in a square hole made perfectly
air-tight, the excess of the external air, to the
inside of the bore of the cylinder, was effectually prevented.
I did not find, however, by this experiment
that the exclusion of the air diminished in the smallest
degree the quantity of heat excited by the friction.
"There still remained one doubt, which, though it
appeared to me to be so slight as hardly to deserve any
attention, I was, however, desirous to remove. The
piston which choked the mouth of the bore of the cylinder,
in order that it might be air-tight, was fitted into
it with so much nicety, by means of its collars of leather,
and pressed against it with so much force, that,
notwithstanding its being oiled, it occasioned a considerable
degree of friction when the hollow cylinder was
turned round its axis. Was not the heat produced, or
at least some part of it, occasioned by this friction of
the piston? and, as the external air had free access to
the extremity of the bore, where it came into contact
with the piston, is it not possible that this air may have
had some share in the generation of the heat produced?
"A quadrangular oblong deal box, water-tight, being
provided with holes or slits in the middle of each of its
ends, just large enough to receive, the one the square
iron rod to the end of which the blunt steel borer was
fastened, the other the small cylindrical neck which
joined the hollow cylinder to the cannon; when this
box (which was occasionally closed above by a wooden
cover or lid moving on hinges) was put into its place--
that is to say, when, by means of the two vertical opening
or slits in its two ends, the box was fixed to the
machinery in such a manner that its bottom being in
the plane of the horizon, its axis coincided with the
axis of the hollow metallic cylinder, it is evident,
from the description, that the hollow, metallic cylinder
would occupy the middle of the box, without touching
it on either side; and that, on pouring water into the
box and filling it to the brim, the cylinder would be
completely covered and surrounded on every side by
that fluid. And, further, as the box was held fast by
the strong, square iron rod which passed in a square
hole in the centre of one of its ends, while the round or
cylindrical neck which joined the hollow cylinder to
the end of the cannon could turn round freely on its
axis in the round hole in the centre of the other end of
it, it is evident that the machinery could be put in
motion without the least danger of forcing the box out
of its place, throwing the water out of it, or deranging
any part of the apparatus."
Everything being thus ready, the box was filled with
cold water, having been made water-tight by means of
leather collars, and the machinery put in motion.
"The result of this beautiful experiment," says Rumford,
"was very striking, and the pleasure it afforded
me amply repaid me for all the trouble I had had in
contriving and arranging the complicated machinery
used in making it. The cylinder, revolving at the rate
of thirty-two times in a minute, had been in motion
but a short time when I perceived, by putting my
hand into the water and touching the outside of the
cylinder, that heat was generated, and it was not long
before the water which surrounded the cylinder began
to be sensibly warm.
"At the end of one hour I found, by plunging a thermometer
into the box, . . . that its temperature had
been raised no less than forty-seven degrees Fahrenheit,
being now one hundred and seven degrees Fahrenheit.
... One hour and thirty minutes after the machinery
had been put in motion the heat of the water in the
box was one hundred and forty-two degrees. At the
end of two hours ... it was raised to one hundred
and seventy-eight degrees; and at two hours and
thirty minutes it ACTUALLY BOILED!
"It would be difficult to describe the surprise and
astonishment expressed in the countenances of the bystanders
on seeing so large a quantity of cold water
heated, and actually made to boil, without any fire.
Though there was, in fact, nothing that could justly be
considered as a surprise in this event, yet I acknowledge
fairly that it afforded me a degree of childish
pleasure which, were I ambitious of the reputation of
a GRAVE PHILOSOPHER, I ought most certainly rather to
hide than to discover...."
Having thus dwelt in detail on these experiments,
Rumford comes now to the all-important discussion as
to the significance of them--the subject that had been
the source of so much speculation among the philosophers--
the question as to what heat really is, and if
there really is any such thing (as many believed) as an
igneous fluid, or a something called caloric.
"From whence came this heat which was continually
given off in this manner, in the foregoing experiments?"
asks Rumford. "Was it furnished by the small particles
of metal detached from the larger solid masses
on their being rubbed together? This, as we have already
seen, could not possibly have been the case.
"Was it furnished by the air? This could not have
been the case; for, in three of the experiments, the machinery
being kept immersed in water, the access
of the air of the atmosphere was completely prevented.
"Was it furnished by the water which surrounded
the machinery? That this could not have been the
case is evident: first, because this water was continually
RECEIVING heat from the machinery, and could not, at
the same time, be GIVING TO and RECEIVING HEAT FROM the
same body; and, secondly, because there was no chemical
decomposition of any part of this water. Had any
such decomposition taken place (which, indeed, could
not reasonably have been expected), one of its component
elastic fluids (most probably hydrogen) must, at
the same time, have been set at liberty, and, in making
its escape into the atmosphere, would have been detected;
but, though I frequently examined the water
to see if any air-bubbles rose up through it, and had
even made preparations for catching them if they
should appear, I could perceive none; nor was there
any sign of decomposition of any kind whatever, or
other chemical process, going on in the water.
"Is it possible that the heat could have been supplied
by means of the iron bar to the end of which the
blunt steel borer was fixed? Or by the small neck of
gun-metal by which the hollow cylinder was united to
the cannon? These suppositions seem more improbable
even than either of the before-mentioned; for heat
was continually going off, or OUT OF THE MACHINERY, by
both these passages during the whole time the experiment
lasted.
"And in reasoning on this subject we must not forget
to consider that most remarkable circumstance,
that the source of the heat generated by friction in
these experiments appeared evidently to be INEXHAUSTIBLE.
"It is hardly necessary to add that anything which
any INSULATED body, or system of bodies, can continue
to furnish WITHOUT LIMITATION cannot possibly be a MATERIAL
substance; and it appears to me to be extremely
difficult, if not quite impossible, to form any distinct
idea of anything capable of being excited and communicated,
in the manner the heat was excited and communicated
in these experiments, except in MOTION."[1]
THOMAS YOUNG AND THE WAVE THEORY OF LIGHT
But contemporary judgment, while it listened respectfully
to Rumford, was little minded to accept his
verdict. The cherished beliefs of a generation are not
to be put down with a single blow. Where many minds
have a similar drift, however, the first blow may precipitate
a general conflict; and so it was here. Young
Humphry Davy had duplicated Rumford's experiments,
and reached similar conclusions; and soon others
fell into line. Then, in 1800, Dr. Thomas Young--
"Phenomenon Young" they called him at Cambridge,
because he was reputed to know everything--took up
the cudgels for the vibratory theory of light, and it
began to be clear that the two "imponderables," heat
and light, must stand or fall together; but no one as
yet made a claim against the fluidity of electricity.
Before we take up the details of the assault made by
Young upon the old doctrine of the materiality of light,
we must pause to consider the personality of Young
himself. For it chanced that this Quaker physician
was one of those prodigies who come but few times in
a century, and the full list of whom in the records of
history could be told on one's thumbs and fingers. His
biographers tell us things about him that read like the
most patent fairy-tales. As a mere infant in arms he
had been able to read fluently. Before his fourth
birthday came he had read the Bible twice through, as
well as Watts's Hymns--poor child!--and when seven
or eight he had shown a propensity to absorb languages
much as other children absorb nursery tattle and Mother
Goose rhymes. When he was fourteen, a young lady
visiting the household of his tutor patronized the pretty
boy by asking to see a specimen of his penmanship.
The pretty boy complied readily enough, and mildly rebuked
his interrogator by rapidly writing some sentences
for her in fourteen languages, including such as,
Arabian, Persian, and Ethiopic.
Meantime languages had been but an incident in the
education of the lad. He seems to have entered every
available field of thought--mathematics, physics, botany,
literature, music, painting, languages, philosophy,
archaeology, and so on to tiresome lengths--and once
he had entered any field he seldom turned aside until he
had reached the confines of the subject as then known
and added something new from the recesses of his own
genius. He was as versatile as Priestley, as profound
as Newton himself. He had the range of a mere dilettante,
but everywhere the full grasp of the master. He
took early for his motto the saying that what one man
has done, another man may do. Granting that the
other man has the brain of a Thomas Young, it is a
true motto.
Such, then, was the young Quaker who came to
London to follow out the humdrum life of a practitioner of
medicine in the year 1801. But incidentally the young
physician was prevailed upon to occupy the interims
of early practice by fulfilling the duties of the chair of
Natural Philosophy at the Royal Institution, which
Count Rumford had founded, and of which Davy was
then Professor of Chemistry--the institution whose
glories have been perpetuated by such names as Faraday
and Tyndall, and which the Briton of to-day
speaks of as the "Pantheon of Science." Here it was
that Thomas Young made those studies which have
insured him a niche in the temple of fame not far removed
from that of Isaac Newton.
As early as 1793, when he was only twenty, Young
had begun to Communicate papers to the Royal Society
of London, which were adjudged worthy to be printed
in full in the Philosophical Transactions; so it is not
strange that he should have been asked to deliver the
Bakerian lecture before that learned body the very first
year after he came to London. The lecture was delivered
November 12, 1801. Its subject was "The
Theory of Light and Colors," and its reading marks
an epoch in physical science; for here was brought forward
for the first time convincing proof of that undulatory
theory of light with which every student of
modern physics is familiar--the theory which holds
that light is not a corporeal entity, but a mere pulsation
in the substance of an all-pervading ether, just as
sound is a pulsation in the air, or in liquids or solids.
Young had, indeed, advocated this theory at an
earlier date, but it was not until 1801 that he hit upon
the idea which enabled him to bring it to anything
approaching a demonstration. It was while pondering
over the familiar but puzzling phenomena of colored
rings into which white light is broken when reflected
from thin films--Newton's rings, so called--that an
explanation occurred to him which at once put the entire
undulatory theory on a new footing. With that sagacity
of insight which we call genius, he saw of a sudden
that the phenomena could be explained by supposing
that when rays of light fall on a thin glass, part of the
rays being reflected from the upper surface, other rays,
reflected from the lower surface, might be so retarded
in their course through the glass that the two sets
would interfere with one another, the forward pulsation
of one ray corresponding to the backward pulsation
of another, thus quite neutralizing the effect.
Some of the component pulsations of the light being
thus effaced by mutual interference, the remaining
rays would no longer give the optical effect of white
light; hence the puzzling colors.
Here is Young's exposition of the subject:
Of the Colors of Thin Plates
"When a beam of light falls upon two refracting
surfaces, the partial reflections coincide perfectly in
direction; and in this case the interval of retardation
taken between the surfaces is to their radius as twice
the cosine of the angle of refraction to the radius.
"Let the medium between the surfaces be rarer than
the surrounding mediums; then the impulse reflected
at the second surface, meeting a subsequent undulation
at the first, will render the particles of the rarer
medium capable of wholly stopping the motion of the
denser and destroying the reflection, while they themselves
will be more strongly propelled than if they had
been at rest, and the transmitted light will be increased.
So that the colors by reflection will be destroyed, and
those by transmission rendered more vivid, when the
double thickness or intervals of retardation are any
multiples of the whole breadth of the undulations; and
at intermediate thicknesses the effects will be reversed
according to the Newtonian observation.
"If the same proportions be found to hold good with
respect to thin plates of a denser medium, which is,
indeed, not improbable, it will be necessary to adopt
the connected demonstrations of Prop. IV., but, at any
rate, if a thin plate be interposed between a rarer and
a denser medium, the colors by reflection and transmission
may be expected to change places.
Of the Colors of Thick Plates
"When a beam of light passes through a refracting
surface, especially if imperfectly polished, a portion of
it is irregularly scattered, and makes the surface visible
in all directions, but most conspicuously in directions
not far distant from that of the light itself; and if
a reflecting surface be placed parallel to the refracting
surface, this scattered light, as well as the principal
beam, will be reflected, and there will be also a new
dissipation of light, at the return of the beam through
the refracting surface. These two portions of scattered
light will coincide in direction; and if the surfaces
be of such a form as to collect the similar effects, will
exhibit rings of colors. The interval of retardation is
here the difference between the paths of the principal
beam and of the scattered light between the two surfaces;
of course, wherever the inclination of the scattered
light is equal to that of the beam, although in
different planes, the interval will vanish and all the
undulations will conspire. At other inclinations, the
interval will be the difference of the secants from the
secant of the inclination, or angle of refraction of the
principal beam. From these causes, all the colors of
concave mirrors observed by Newton and others are
necessary consequences; and it appears that their production,
though somewhat similar, is by no means as
Newton imagined, identical with the production of
thin plates."[2]
By following up this clew with mathematical precision,
measuring the exact thickness of the plate and
the space between the different rings of color, Young
was able to show mathematically what must be the
length of pulsation for each of the different colors of the
spectrum. He estimated that the undulations of red
light, at the extreme lower end of the visible spectrum,
must number about thirty-seven thousand six hundred
and forty to the inch, and pass any given spot at a rate
of four hundred and sixty-three millions of millions of
undulations in a second, while the extreme violet numbers
fifty-nine thousand seven hundred and fifty undulations
to the inch, or seven hundred and thirty-five
millions of millions to the second.
The Colors of Striated Surfaces
Young similarly examined the colors that are produced
by scratches on a smooth surface, in particular
testing the light from "Mr. Coventry's exquisite micrometers,"
which consist of lines scratched on glass at
measured intervals. These microscopic tests brought
the same results as the other experiments. The colors
were produced at certain definite and measurable
angles, and the theory of interference of undulations
explained them perfectly, while, as Young affirmed
with confidence, no other hypothesis hitherto advanced
would explain them at all. Here are his
words:
"Let there be in a given plane two reflecting points
very near each other, and let the plane be so situated
that the reflected image of a luminous object seen in it
may appear to coincide with the points; then it is obvious
that the length of the incident and reflected ray,
taken together, is equal with respect to both points,
considering them as capable of reflecting in all directions.
Let one of the points be now depressed below
the given plane; then the whole path of the light reflected
from it will be lengthened by a line which is to
the depression of the point as twice the cosine of incidence
to the radius.
"If, therefore, equal undulations of given dimensions
be reflected from two points, situated near enough to
appear to the eye but as one, whenever this line is equal
to half the breadth of a whole undulation the reflection
from the depressed point will so interfere with the reflection
from the fixed point that the progressive motion
of the one will coincide with the retrograde motion
of the other, and they will both be destroyed; but
when this line is equal to the whole breadth of an
undulation, the effect will be doubled, and when to a
breadth and a half, again destroyed; and thus for a
considerable number of alternations, and if the reflected
undulations be of a different kind, they will be
variously affected, according to their proportions to
the various length of the line which is the difference
between the lengths of their two paths, and which may
be denominated the interval of a retardation.
"In order that the effect may be the more perceptible,
a number of pairs of points must be united into
two parallel lines; and if several such pairs of lines be
placed near each other, they will facilitate the
observation. If one of the lines be made to revolve
round the other as an axis, the depression below the
given plane will be as the sine of the inclination; and
while the eye and the luminous object remain fixed
the difference of the length of the paths will vary as
this sine.
"The best subjects for the experiment are Mr. Coventry's
exquisite micrometers; such of them as consist
of parallel lines drawn on glass, at a distance of one-
five-hundredth of an inch, are the most convenient.
Each of these lines appears under a microscope to consist
of two or more finer lines, exactly parallel, and at a
distance of somewhat more than a twentieth more than
the adjacent lines. I placed one of these so as to reflect
the sun's light at an angle of forty-five degrees,
and fixed it in such a manner that while it revolved
round one of the lines as an axis, I could measure its
angular motion; I found that the longest red color
occurred at the inclination 10 1/4 degrees, 20 3/4 degrees, 32
degrees, and 45 degrees; of
which the sines are as the numbers 1, 2, 3, and 4. At
all other angles also, when the sun's light was reflected
from the surface, the color vanished with the inclination,
and was equal at equal inclinations on either side.
This experiment affords a very strong confirmation
of the theory. It is impossible to deduce any explanation
of it from any hypothesis hitherto advanced;
and I believe it would be difficult to invent any other
that would account for it. There is a striking analogy
between this separation of colors and the production
of a musical note by successive echoes from equidistant
iron palisades, which I have found to correspond pretty
accurately with the known velocity of sound and the
distances of the surfaces.
"It is not improbable that the colors of the integuments
of some insects, and of some other natural bodies,
exhibiting in different lights the most beautiful
versatility, may be found to be of this description, and
not to be derived from thin plates. In some cases a
single scratch or furrow may produce similar effects,
by the reflection of its opposite edges."[3]
This doctrine of interference of undulations was the
absolutely novel part of Young's theory. The all-
compassing genius of Robert Hooke had, indeed, very
nearly apprehended it more than a century before, as
Young himself points out, but no one else bad so much
as vaguely conceived it; and even with the sagacious
Hooke it was only a happy guess, never distinctly outlined
in his own mind, and utterly ignored by all others.
Young did not know of Hooke's guess until he himself
had fully formulated the theory, but he hastened then
to give his predecessor all the credit that could possibly
be adjudged his due by the most disinterested observer.
To Hooke's contemporary, Huygens, who was the
originator of the general doctrine of undulation as the
explanation of light, Young renders full justice also.
For himself he claims only the merit of having demonstrated
the theory which these and a few others of his
predecessors had advocated without full proof.
The following year Dr. Young detailed before the
Royal Society other experiments, which threw additional
light on the doctrine of interference; and in 1803
he cited still others, which, he affirmed, brought the
doctrine to complete demonstration. In applying this
demonstration to the general theory of light, he made
the striking suggestion that "the luminiferous ether
pervades the substance of all material bodies with little
or no resistance, as freely, perhaps, as the wind passes
through a grove of trees." He asserted his belief also
that the chemical rays which Ritter had discovered
beyond the violet end of the visible spectrum are but
still more rapid undulations of the same character as
those which produce light. In his earlier lecture he
had affirmed a like affinity between the light rays and
the rays of radiant heat which Herschel detected below
the red end of the spectrum, suggesting that "light
differs from heat only in the frequency of its undulations
or vibrations--those undulations which are
within certain limits with respect to frequency affecting
the optic nerve and constituting light, and those
which are slower and probably stronger constituting
heat only." From the very outset he had recognized
the affinity between sound and light; indeed, it had
been this affinity that led him on to an appreciation
of the undulatory theory of light.
But while all these affinities seemed so clear to the
great co-ordinating brain of Young, they made no such
impression on the minds of his contemporaries. The
immateriality of light had been substantially demonstrated,
but practically no one save its author accepted
the demonstration. Newton's doctrine of the emission
of corpuscles was too firmly rooted to be readily dislodged,
and Dr. Young had too many other interests to
continue the assault unceasingly. He occasionally
wrote something touching on his theory, mostly papers
contributed to the Quarterly Review and similar periodicals,
anonymously or under pseudonym, for he had
conceived the notion that too great conspicuousness in
fields outside of medicine would injure his practice as a
physician. His views regarding light (including the
original papers from the Philosophical Transactions of
the Royal Society) were again given publicity in full in
his celebrated volume on natural philosophy, consisting
in part of his lectures before the Royal Institution, published
in 1807; but even then they failed to bring conviction
to the philosophic world. Indeed, they did not
even arouse a controversial spirit, as his first papers
had done.
ARAGO AND FRESNEL CHAMPION THE WAVE THEORY
So it chanced that when, in 1815, a young French
military engineer, named Augustin Jean Fresnel, returning
from the Napoleonic wars, became interested
in the phenomena of light, and made some experiments
concerning diffraction which seemed to him to controvert
the accepted notions of the materiality of light,
he was quite unaware that his experiments had been
anticipated by a philosopher across the Channel. He
communicated his experiments and results to the
French Institute, supposing them to be absolutely
novel. That body referred them to a committee, of
which, as good fortune would have it, the dominating
member was Dominique Francois Arago, a man as versatile
as Young himself, and hardly less profound, if
perhaps not quite so original. Arago at once recognized
the merit of Fresnel's work, and soon became a
convert to the theory. He told Fresnel that Young
had anticipated him as regards the general theory, but
that much remained to be done, and he offered to associate
himself with Fresnel in prosecuting the investigation.
Fresnel was not a little dashed to learn that
his original ideas had been worked out by another
while he was a lad, but he bowed gracefully to the
situation and went ahead with unabated zeal.
The championship of Arago insured the undulatory
theory a hearing before the French Institute, but by no
means sufficed to bring about its general acceptance.
On the contrary, a bitter feud ensued, in which Arago
was opposed by the "Jupiter Olympus of the Academy,"
Laplace, by the only less famous Poisson, and by
the younger but hardly less able Biot. So bitterly
raged the feud that a life-long friendship between
Arago and Biot was ruptured forever. The opposition
managed to delay the publication of Fresnel's papers,
but Arago continued to fight with his customary enthusiasm
and pertinacity, and at last, in 1823, the
Academy yielded, and voted Fresnel into its ranks,
thus implicitly admitting the value of his work.
It is a humiliating thought that such controversies as
this must mar the progress of scientific truth; but fortunately
the story of the introduction of the undulatory
theory has a more pleasant side. Three men, great both
in character and in intellect, were concerned in pressing
its claims--Young, Fresnel, and Arago--and the relations
of these men form a picture unmarred by any
of those petty jealousies that so often dim the lustre
of great names. Fresnel freely acknowledged Young's
priority so soon as his attention was called to it; and
Young applauded the work of the Frenchman, and
aided with his counsel in the application of the undulatory
theory to the problems of polarization of light,
which still demanded explanation, and which Fresnel's
fertility of experimental resource and profundity
of mathematical insight sufficed in the end to
conquer.
After Fresnel's admission to the Institute in 1823
the opposition weakened, and gradually the philosophers
came to realize the merits of a theory which
Young had vainly called to their attention a full quarter-
century before. Now, thanks largely to Arago, both
Young and Fresnel received their full meed of appreciation.
Fresnel was given the Rumford medal of the
Royal Society of England in 1825, and chosen one of
the foreign members of the society two years later,
while Young in turn was elected one of the eight foreign
members of the French Academy. As a fitting culmination
of the chapter of felicities between the three
friends, it fell to the lot of Young, as Foreign Secretary
of the Royal Society, to notify Fresnel of the honors
shown him by England's representative body of scientists;
while Arago, as Perpetual Secretary of the French
Institute, conveyed to Young in the same year the notification
that he had been similarly honored by the
savants of France.
A few months later Fresnel was dead, and Young
survived him only two years. Both died prematurely,
but their great work was done, and the world will remember
always and link together these two names in
connection with a theory which in its implications and
importance ranks little below the theory of universal
gravitation.
VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM
GALVANI AND VOLTA
The full importance of Young's studies of light
might perhaps have gained earlier recognition
had it not chanced that, at the time when they were
made, the attention of the philosophic world was turned
with the fixity and fascination of a hypnotic stare
upon another field, which for a time brooked no rival.
How could the old, familiar phenomenon, light, interest
any one when the new agent, galvanism, was in view?
As well ask one to fix attention on a star while a meteorite
blazes across the sky.
Galvanism was so called precisely as the Roentgen
ray was christened at a later day--as a safe means of
begging the question as to the nature of the phenomena
involved. The initial fact in galvanism was the discovery
of Luigi Galvani (1737-1798), a physician of
Bologna, in 1791, that by bringing metals in contact
with the nerves of a frog's leg violent muscular contractions
are produced. As this simple little experiment
led eventually to the discovery of galvanic electricity
and the invention of the galvanic battery, it
may be regarded as the beginning of modern electricity.
The story is told that Galvani was led to his discovery
while preparing frogs' legs to make a broth for his
invalid wife. As the story runs, he had removed the
skins from several frogs' legs, when, happening to touch
the exposed muscles with a scalpel which had lain in
close proximity to an electrical machine, violent muscular
action was produced. Impressed with this phenomenon,
he began a series of experiments which finally
resulted in his great discovery. But be this story authentic
or not, it is certain that Galvani experimented
for several years upon frogs' legs suspended upon wires
and hooks, until he finally constructed his arc of two
different metals, which, when arranged so that one was
placed in contact with a nerve and the other with a
muscle, produced violent contractions.
These two pieces of metal form the basic principle of
the modern galvanic battery, and led directly to Alessandro
Volta's invention of his "voltaic pile," the immediate
ancestor of the modern galvanic battery.
Volta's experiments were carried on at the same time
as those of Galvani, and his invention of his pile followed
close upon Galvani's discovery of the new form
of electricity. From these facts the new form of electricity
was sometimes called "galvanic" and sometimes
"voltaic" electricity, but in recent years the
term "galvanism" and "galvanic current" have almost
entirely supplanted the use of the term voltaic.
It was Volta who made the report of Galvani's wonderful
discovery to the Royal Society of London, read
on January 31, 1793. In this letter he describes Galvani's
experiments in detail and refers to them in
glowing terms of praise. He calls it one of the "most
beautiful and important discoveries," and regarded it
as the germ or foundation upon which other discoveries
were to be made. The prediction proved entirely correct,
Volta himself being the chief discoverer.
Working along lines suggested by Galvani's discovery,
Volta constructed an apparatus made up of a
number of disks of two different kinds of metal, such
as tin and silver, arranged alternately, a piece of some
moist, porous substance, like paper or felt, being interposed
between each pair of disks. With this "pile,"
as it was called, electricity was generated, and by linking
together several such piles an electric battery could
be formed.
This invention took the world by storm. Nothing
like the enthusiasm it created in the philosophic world
had been known since the invention of the Leyden jar,
more than half a century before. Within a few weeks
after Volta's announcement, batteries made according
to his plan were being experimented with in every
important laboratory in Europe.
As the century closed, half the philosophic world
was speculating as to whether "galvanic influence"
were a new imponderable, or only a form of electricity;
and the other half was eagerly seeking to discover
what new marvels the battery might reveal. The
least imaginative man could see that here was an
invention that would be epoch-making, but the most
visionary dreamer could not even vaguely adumbrate
the real measure of its importance.
It was evident at once that almost any form of galvanic
battery, despite imperfections, was a more satisfactory
instrument for generating electricity than the
frictional machine hitherto in use, the advantage lying
in the fact that the current from the galvanic battery
could be controlled practically at will, and that the
apparatus itself was inexpensive and required
comparatively little attention. These advantages were
soon made apparent by the practical application of the
electric current in several fields.
It will be recalled that despite the energetic endeavors
of such philosophers as Watson, Franklin, Galvani,
and many others, the field of practical application of
electricity was very limited at the close of the
eighteenth century. The lightning-rod had come into
general use, to be sure, and its value as an invention
can hardly be overestimated. But while it was the
result of extensive electrical discoveries, and is a most
practical instrument, it can hardly be called one that
puts electricity to practical use, but simply acts as a
means of warding off the evil effects of a natural
manifestation of electricity. The invention, however, had
all the effects of a mechanism which turned electricity
to practical account. But with the advent of the new
kind of electricity the age of practical application began.
DAVY AND ELECTRIC LIGHT
Volta's announcement of his pile was scarcely two
months old when two Englishmen, Messrs. Nicholson
and Carlisle, made the discovery that the current from
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