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96 to 100 degrees, which happened in the experiments, rather to the acceleration of the blood, than to the influx of heat from the external air. While the caufe of animal heat remains unknown, it would be prefumption to affert, that these are the only means by which the body is enabled to relift the effects of external heat. There may be others; and it is not unreasonable to fuppofe, that as external cold, perhaps by its tonic influence, increases the power of the body to generate heat, fo external heat may diminish that power, and thus leffen the quantity of heat generated within, while the evaporation, produced by the fame caufe, guards it against receiving any acceflion from without."

AN ESSAY on the ASCENT of VAPOUR, BY ALEXANDER EASON, M. D.

[From the fame Publication. ]

"THE

"THERE are few phenomena in nature, which have puzzled philofophers more, than the afcent of vapour; and the different theories laid down by doctors Halley and Defaguliers, have been rejected, while another, not lefs liable to objections, has been almost univerfally received.

This theory, which I fhall presently mention, was at first invented by a French gentleman, Monfieur le Roi, and afterwards revived by Lord Kaimes, and doctor Hugh Hamilton. It is this that the air diffolves water, as water does faline fubftances: the folution being perfect, the air will become transparent. Objections. 1. Were this theory true, evaporation could not be

performed without air; but Mr. Watt, contrary to the theory fup. ported by Lord Kaimes and Dr. Has milton, has proved, that when was ter in vacuo was boiled with a degree of heat very little greater than that of the human body, the steam came over, and was condenfed in the refrigeratory. But he relates, that the evaporation was not quicker than in the open air.

"2. Were the doctrine of folution true, the air would be heavier, the more water it contained; and, as clouds contain a great portion of water, they ought to float on the furface of the earth, and not in the higher regions, as we daily obferve,

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3. We never could expect any rain, unleis the air were fuperiatu

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rated with water; and it would only vield to us, what it could not retain in folution..

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4. It is univerfally allowed, that heat contributes very much towards converting water into vapour, which is again condenfed by cold. In what manner will the doctrine of folution account for the fpontaneous evaporation of water, and its being fufpended in air, in the coldeft weather, even when the thermometer is below the freezing point? Though I cannot allow of fuch a folution as above mentioned, I can, however, readily admit of a strong attraction betwixt air and water: for no air is found without water, and no water

without air.

"Water, which is eight hundred times heavier than air, by a very fmall degree of heat may be converted into vapour, which vapour is one thoufand eight hundred times lighter than air, according to Mr. Watt. It confequently follows, that vapour will rife up in the atmofphere, to the height of its own fpecific gravity; but, long before it could reach to fo high a region, it would be condensed by cold, and return to the earth in rain, were it not for the latent heat it contains, and the electric matter in the air. "Whatever I mention concerning electricity is from facts, and not from any theory written about it, which is above my comprehenfion. But as the terms now in ufe, viz. pofitive and negative, or plus and minus, are generally beft underftood, I fhall exprefs myfelf by them. The able Nollet has proved, that water electrified, will evaporate fafter, than water which is not elec. trified. Does is not follow, that the more electric matter is in the air, the quicker the evaporation of water will be? And Mr. Cavallo has proved, that at all times the at

mofphere is electrified, but much. ftronger in frofty, than in warm weather, and by no means less in the night than in the day it is likewife ftronger in elevated than in low places. From these facts we may be enabled to account, why evaporation is carried on during very cold weather. All the heat contained in water, above what is fufficient to keep it in a fluid state, will convert it into vapour; which, in a north or north-east wind, when the electric matter greatly abounds, will be carried off with much rapi dity; and, by the power of electricity, will be rendered ftill lighter, the higher it afcends; each particle repelling each other, and prevent. ing the cold from condenfing the vapour, in its afcent through the cold regions of the atmosphere. The higher it rifes, the more space there is for expansion; and the more it is expanded, the clearer will the atmosphere appear, and, probably, the higher the mercury will rife in the barometer.

"It likewife appears, that the electric matter is more fenfible near the furface of the earth, in cold northern countries, than in warm fouthern places. M. Volta, with a very fimple apparatus, on the upper gallery of St. Paul's, produced an electric fpark, which, he told me, in Italy, could not be done, but on a very high mountain, or in a fituation greatly elevated. This feems a wife provifion in nature, that the electric matter fhould ap pear near the furface of the earth in cold climates, to raife up and fufpend the vapour in the air, which otherways would be condenfed by the cold; whereas, in warm coun tries, the heat of the earth will be fufficient to raise vapours to a great height, which are afterwards carried ftill higher, by the electric mat

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ter in the upper regions. This, perhaps, is the caufe, why the air is fo clear and transparent in warm climates.

"By making fome obfervations on the falling of rain, we fhall have other proofs, that the electric mat ter is the great caufe by which vapour is fupported in the atmosphere. Here I muft obferve a fact, well known to all prefent, that bodies electrified, by the fame electric power (no matter whether pofitive or negative) repel each other; and, when electrified by the different powers, that is, the one plus and the other minus, attract each other: on coming into contact, an equilibrium is restored, and neither of them will fhew any figns of electricity.

"From this it follows: if two clouds are electrified by the fame power, they will repel each other, and the vapour be fufpended in both; but, when one is pofitive and the other negative, they will attract each other, and restore an equilibrium. The electric power, by which the vapour was fufpended, being now destroyed by the mutual action of the clouds on each other, the particles of water will have an opportunity of running together into each other, and, as they augment in fize, will gain a greater degree of gravity, defcending in fmall rain, or a heavy thower, according to circumstances.

"A cloud, highly electrified, paffing over a high building or mountain, may be attracted by, and be deprived of its electricity, without or with a violent explosion of thunder. If the cloud is electrified plus, the fire will defcend from the cloud to the mountain; but, if it be electrified minus, the fire will afçend from the mountain to the cloud. In both cafes, the effect is

the fame, and generally, heavy rain immediately, or foon after, follows: this is well known to the inhabitants of, and travellers among, moun tains.

"From this we can cafily ac count, why thunder-fhowers are often partial, falling near, or among mountains, and the rain in fuch quantities, as to occafion rivers to be overflowed; whilft, at the di ftance of a few miles, the ground continues parched up with drought, and the roads covered with duit.

"It often happens, that one clap of thunder is not fufficient to produce rain from a cloud, nor even a fecond: in fhort, the claps must be repeated, till an equilibrium is reftored, and then the rain muft, of confequence, fall. Sometimes we may have violent thunder and lightning without rain, and the black appearance of the heavens may be changed to a clear tranfparent ky, especially in warm weather. To account for this, it must be remem bered, as I lately faid, that one or more claps of thunder are not always fufficient to produce rain from the clouds: fo, if an equilibrium be not restored, little or no rain will fall, and in a fhort time the electric matter, paffing from the earth to the clouds, or the fuperabundant quantity in the air, will electrify thofe black clouds, by which means the particles of vapour will be expanded, raifed higher, and the air become clear. Clouds may be melted away, even when we are looking at them, by another caufe, that is, by the heat of the fun. We know, that tranfparent bodies are not heated by the fun, but opaque ones are ; the clouds being opaque bodies, are warmed by the rays of the fun fhining on them, and any additional quantity of heat wili rarify the va pour, and occafion its expanding in

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the air, which will foon become
transparent. When vapour is made
to expand more than it would o-
therwife do, a certain quantity of
abfolute heat is neceflary to keep it
in the form of vapour; therefore,
when the receiver of an air-pump
is exhaufting, it appears muddy,
and a number of drops are found
within it; the moisture contained
in the air, in the form of vapour,
being made to occupy a greater
fpace than what is natural to it, and
receiving no addition of heat, a
part of it is condensed.

"If, therefore, the air is fud-
denly rarified, a few drops of rain
will defcend, as may often be ob-
ferved in the fummer season.

"I have repeatedly obferved, efpecially during the fummer, when the wind is at north-east, that the weather is, in general, cold and dry, with a clear atmosphere. Should the wind fuddenly change to south-west, in a few hours, black clouds begin to gather, vegetables look fickly, and droop their leaves; and, foon after, comes on a violent ftorm of thunder, with heavy rain. "This change, I imagine, is not fo much owing to the fouth-west wind bringing rain, as to the atmofphere's being changed from an electric ftate, capable of fufpending vapour, to a ftate of parting with its moisture. As foon as the ftorm is going off, vegetables revive from their languid ftate, and the air recovers its ufual afpect. From this we may conclude, that no inftrument can be made to ascertain the quantity of moisture in the air: all that is, or ought to be expected from a hygrometer, is to fhew, whether the air be in a state to retain or part with its moisture. In apparent dry weather it may point to rain; and when it rains, it may

point to fair. For this reafon, the ftones of halls, and fmooth fubftances, are often bedewed with wet, in dry warm weather (that is, the airs in a state to part with its moifture), and, vite verfa, they will dry in the time of rain.

"Left this paper should exceed the common limits of time in reading, I fhall pafs over thofe obfervations, which might be made on fogs or mifts; a few excepted, which I fhall here fubjoin.

"Fogs are produced by two caufes as different as their effects are oppofite. A fog may be produced by a precipitation of rain, in very fmall particles, like a cloud floating on the furface of the earth. In this cafe the air is moist and damp, and never fails to wet a traveller's cloaths; the ftones of the street, painted doors, and hard, cool, fmooth bodies, are generally covered with moisture, which often runs in large drops: this, I dare fay, has been obferved by every perfon. Secondly, a fog may be produced by the abforption of moisture, when the air is too dry, and differs from the other juft defcribed; for it will not impart any of its moisture even to dry bodies; no damp is to be met with on ftones, polifhed marble, &c. This fact is well known to the inhabitants on the fea-coaft of Fifefhire, who, during their fummer months, have frequent opportunities of obferving a fog in the afternoon, driving up the Firth of Forth, with a drying east wind, which of ten blasts the trees and young vegetables, and, therefore, in a small degree, refembles the Harmattan in drying up the ground, and robbing vegetables of their moisture.

"I fhall now conclude with a fhort fummary of the whole. "1. That heat is the great cause,

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by which water is converted into vapour, which is condenfed by cold.

46 2.

That electricity renders vapour fpecifically lighter, and adds to its abfolute heat, repelling its particles; which particles would be condenfed by cold: and that electricity is the great agent by which vapour afcends to the upper regions.

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3. That when the electric power, by which vapour is fufpended in the atmosphere, is dettroyed, a heavy miit, imall rain, or thunder-fhowers, will be the confequence. Had the advocates for the doctrine of folution made heat and electricity the folvents, their theory would have been lefs exceptionable."

ANTI

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