A History of Science - v.3 (Williams)

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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

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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

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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"

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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

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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.

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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

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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

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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,

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"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.

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