Графични страници
PDF файл
ePub

fure, and clearly fhews that they afford no foundation for that gentleman's hypothefis, of the rays of the fun being, in them-' felves, fire.

M. de Sauffure fuppofes that the cold on the prominent tops of mountains is owing to the air that furrounds them being incapable of receiving much heat from the rays of the fun, on account of its own transparency, or from the earth, on account of its diftance. To try whether the direct rays of the fun would have the fame influence there, as on the plains, on a body defended from the air, he made a wooden box, half an inch thick, lined with plates of blackened cork, an inch thick, and covered at top with three fliders of plate glass above one another, at diftances of an inch and a half. On the top of Cramont, July 16th, the box being gradually warmed in the fun, a thermometer in the bottom rofe to 50°: being then kept with the glafs fide expofed directly to the fun for an exact hour, from 12 minutes paft 2 till 12 minutes paft 3, the thermometer rose to 70°: another thermometer, laid on blackened cork on the outfide of the box, rofe only to 21°, while a third, with its bulb naked, ex-" pofed in the open air to the fun's rays, four feet from the ground, was only at 5. Next day, which happily was a fine one, perfectly fuch as the preceding, the experiment was repeated on the plain, with particular attention that every circumstance should be the fame when the box was warmed in the fun, the thermometer in it rofe as before to 50: by direct exposure to the fun, it rose to 69, or I fhort of what it had been on the top of the mountain; though the thermometer on the outfide rofe 6, and that in the open air 14, higher than they did in the other fituation. Thus an elevation of 777 fathoms produced a diminution of 14 degrees in the heat which the rays of the fun are capable of communicating to a body entirely exposed to the air, a diminution of 6 only in a body partly fheltered from the air, and an increase of one degree in a body entirely fheltered.

We believe moft of our Readers would be apt to conclude from these experiments, as M. de Sauffure has done, that it was the cold air on the mountain which diminished the effect of the fun's heat on the thermometer expofed to it; but M. de Luc confiders them in a different light, and, inftead of being inconfiftent with his theory, finds that they confirm it. The bulb of a mercurial thermometer is not affected at all by the rays of the fun, because it reflects them: it fhews only the temperature of the air or contiguous bodies, a fact which M. de Sauffure himfelf, as well as M. de Luc, have eftablished in fome of their other works. The air on the mountain is leaft heated by the fun, because it is rareft and contains leaft of that matter which forms fire with the light; the cork contains much of that matter, and part of the fire formed in it paffed to the thermometer

[ocr errors]

in contact with it. The coldness of the air on the mountain Occafioned this fire to be diffipated fafter; though the quantity produced was fomewhat greater, on account of the greater denfity of the rays of the fun, as being lefs intercepted by a purer or rarer air.

M. de Sauffure obferves, that the rays of the fun, in paffing through a rare and pure air, heat it but little, and that they produce greater and greater heat in proportion as the air is more denfe and loaded with vapours. Now this very circumftance may be confidered as a proof that light is not fire, for fire would follow a very different progreffion: if a globe of metal, very hot but not luminous, was fufpended in the upper part of the atmosphere, it would heat the parts nearest to it moft, and the earth could receive no greater heat than that of the ftratum of air contiguous to it.

M. de Sauffure endeavours to prove likewife, that the greater heat of the air on the plains is owing to the heat communicated to it by the earth. But M. de Luc has fhewn, from the curious experiments of M. Pictet, that, in the night, to the height of 50 feet, which is as far as thefe experiments went, there is no ftratum of air lefs hot than that which refts immediately on the ground; and how little influence the heat of the earth can have in warming the air, may be judged from fome obfervations made by M. de Luc himself, of hoar froft forming on grass, and the thermometer, when laid on the furface of the grafs, finking below the freezing point, though in the air it was a little above the freezing point, and at the bottom of the grafs feveral degrees higher ftill; the heat of the earth being infufficient to counteract the effect of evaporation, even in the ftratum contiguous to it.

Of the difference between the fun's rays and fire, our own fenfations are produced as an evidence. Every one who has obferved the march of a thermometer in the air must have taken notice how differently the heat produced by the fun affects us, from that which arifes from other causes; but M. de Sauffure gives an account of fome facts, relative to this fubject, more triking than any that had before occurred to us. Speaking of an attempt made by four mountaineers to reach the fummit of Mont Blanc, he fays they went on without impediment till they came to a great valley of fnow that feemed to lead directly to the fummit: The reverberation of the fun upon the fnow, and ftagnation of the air in this valley, made them feel a fuffocating heat, and gave them fuch a difguft to their provifions, that, exhaufted with inanition and fatigue, they had the mortification of being obliged to turn back."-Three others, who made a like attempt, "were marching on courageously, when one of them, the boldest and moft robuft, was feized almoft fuddenly with an

66

3

abfolutely

[ocr errors]

abfolutely infurmountable defire of fleep, which made them give up the enterprize: they were all incommoded exceffively by heat, a thing aftonishing at that height; their appetite left them; they loathed their wine and provifions." In another place he fays, the most infurmountable obftacle met with by thofe who attempted to reach the top of Mont Blanc, has always been the heat of the fun. I fhould have been tempted to doubt an affertion fo ftrange, and fo contrary to the received ideas of the coldness of thofe elevated regions, if the relations had not borne all the characters of truth, and if I had not myself experienced the fame fenfations. During an hour which we paffed at the height of 1900 fathoms, the fun incommoded us to fuch a degree as to appear infupportable when his rays ftruck directly on any part of the body; we could not bear to be out of the fhade of our umbrellas *. Yet thefe rays, fo unfupportable to our bodies, produced an effect on the ball of the thermometer equivalent only to 2 degrees."

This extraordinary fenfation of heat, while there was fo little real heat in the air, and while a fire, lighted in the fhade of fome projecting rock, would probably have been comfortable, is apparently moft favourable to the theory of M. de Luc. M. de Sauffure attributes it to the relaxation and weakness of the animal frame, in the rare atmosphere, from the diminution of external preffure; but though it doubtless depends on the animal ceconomy, its fource muft be of another kind, connected alfo with fome local circumftance as yet unknown. M. de Luc and others have afcended to equal heights without perceiving any inconvenience of this kind. So far are the mountaineers from having any uneasy fenfations at great heights, that they find all bodily exertions, walking in particular, to be easier and more agreeable the higher they afcend; and to this, principally, the Author attributes their delight in hunting the chamois, a kind of life in appearance fo laborious, and attended with fo little profit; but they are content with little on the mountains, because they feel themselves happy there.'

[ocr errors]

From the whole of the obfervations on aereal heat, M. de Luc concludes, that the greater heat which the fun's rays produce in the lower part of the atmosphere is owing, not to the greater denfity of the air, but to the greater quantity of watery vapours it contains. As it is in that part of the day in which the fun produces greatest heat in the lower ftrata, that the watery vapours diminish there, yet without reaching the upper ones, it is natural to conclude, that the two effects are connected with one another, and that it is the fun's rays which produce the transformation of

* Some of our aeronauts alfo, if we remember right, experienced exceffive heat from the direct action of the fun's rays at great heights.

[blocks in formation]

the vapours into air. And as in this cafe a fufficiency of fire must be formed, not only for fupplying what is neceffary to the conftitution of the new air, but likewife for producing an increase in the quantity of free fire; it will follow, that the water, which then difappears, contained the matter of fire, and that the fun's rays, in producing fire with that matter, produce also the fubftance which diftinguishes the nitrous acid.

That water is transformable into air by the fun's light, appears directly, from an experiment of Dr. Prieftley's. Of two equal receivers, containing the fame quantity of water, one was exposed to the fun, and the other kept in the fhade: a quantity of air was collected in the upper part of the former, and on fhaking the water, a multitude of air bubbles appeared through its whole mafs; but nothing of this kind took place in the fhaded receiver, though the heat was the fame in both.

.. The Author is hence led to confider the electricity of the atmofphere; and from the numerous obfervations of M. de Sauffure, and the general laws of aereal electricity deduced from them, of which he has here given an abftract, he shews the production of the electrical fluid to correfpond remarkably with the two above mentioned diurnal phenomena of heat and vapours; and concludes, that this fluid alfo, like fire and air, is generated by the fun's rays; that its generation and decompofition are inceffantly going on; and that its ingredients feparately, or in other combinations, have much more important functions in the ceconomy of nature, than the electrical fluid itfelf in the compound ftate in which it is fenfible to us,

We next meet with fome interefting obfervations on the reconverfion of air into vapour, that is, the formation of clouds and rain; the immediate caufe of which the Author does not pretend to have discovered. To ftop up fome roads that would mislead us in this enquiry, he obferves, that though the cause of the transformation of vapour into air be given, it does not follow that any analogy fhould obtain in the converfe change; and that clouds have no connection with light or with heat, with the time of the day or the feafon of the year. They form in diftinct maffes, in a particular ftratum of the atmosphere, always at a great height, where the air is rare and dry. In the continuance of cloudy or overcaft weather, which fometimes happens over the highest Alps, the caufe of clouds and rain is inceffantly acting, though not in a fufficient degree to counterbalance evaporation. The Author thinks it probable, that they may be produced by fome exhalation from the earth, collected in a ftratum of the atmosphere correfponding to it in gravity.

The theory of clouds and rain lays the foundation of a new theory of winds alfo, which now appear to be, very frequently at least, a directly chemical effect. When a mixture of dephlogifticated

gifticated and inflammable air is converted into water by the electrical fpark, an explosion happens, that is, in becoming va pour, it expands into a larger volume; and this vapour being condensed (inftantaneously in our fmall experiments) into water, a vacuum is left. It will therefore readily occur to the reader, what agitations muft happen in the atmosphere, from the expanfion which accompanies the formation of a cloud, and from the filling up of the immenfe vacuity produced by rain.

. The work concludes with fome general remarks on the nature of caufes and effects in the atmosphere; pointing out the neceffity, and the foundation which the late philofophical difcoveries. afford by analogy, for admitting the exiftence of fubftances as yet unknown to us. Wherever we fee phyfical actions, there must be phyfical agents; and most of thefe agents are, in their uncompounded ftate, fo utterly imperceptible to our fenfes, that we cannot have the leaft intimation of them any other wife than by rational induction from the phenomena.

We have the pleasure of finding at the end of the volume, that the Author is continuing his ingenious labours, and that we may foon expect from him two other works, one directly on hygrometry, the other on miscellaneous philofophical fubjects. Some of his hygrometers have been made by Nairne and Blunt, and Mr. Hurter, and found to correfpond fufficiently with one another, but to differ exceedingly from thofe of M. de Sauffure, thefe laft being apparently erroneous.

Ch

ART. V. Seconde Suite, &c. A Second Continuation of Confiderations on the Mechanifm of Societies. By the Marquis de Cafaux. 8vo. 35. Elmsley. 1787.

WITH the zeal of a writer altogether convinced, himself,

of the rectitude of thofe principles on which his fyftem is founded, the Marquis de Cafaux endeavours, in this feconde fuite, to explain the principal doctrines contained in his original works, in a more familiar, and, to us, in many cafes, a more fatisfactory manner than before; and we doubt not but it will be read with pleasure by many who found lefs entertainment in the first performance. Many pofitions, which, as they were first announced, had too much of a paradoxical appearance, are here very fatisfactorily explained. When we firft perused his Confiderations on the Mechanism of Societies, we thought it a performance remarkably calculated for roufing the mind from that ftate of lethargy into which it is apt to fall, by the deference that men are naturally difpofed to pay to received opinions; and we have now no reafon to alter our judgment in this respect. Though we cannot always affent to the juftnefs of his principles, yet he will be found to be an attentive obferver, and an acute reasoner, on all occafions; fo that his pofitions.deserve to

« ПредишнаНапред »