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To Dr. Percival, Manchester

distance, and the day shortest, but some time Meteorological Imaginations and Conjectures.*

after that period, according to the English

proverb, which says, "as the day lengthens, There seems to be a region higher, in the the cold strengthens ;” the causes of refrigeair over all countries, where it is always win- ration continuing to operate, while the sun ter, where frost exists continually, since in the returns too slowly, and his force continues too midst of summer, on the surface of the earth, weak to counteract them. ice falls often from above in the form of hail. During several of the summer months of the

Hailstones, of the great weight we some- year 1783, when the effects of the sun's rays times find then, did not probably acquire their to heat the earth in these northern regions magnitude before they began to descend. The should have been the greatest, there existed air being eight hundred times rarer than wa- a constant fog over all Europe, and great part ter, is unable to support it but in the shape of of North America. This fog was of a pervapour, a state in which its particles are sepa- manent nature: it was dry, and the rays of rated. As soon as they are condensed by the the sun seemed to have little effect towards cold of the upper region, so as to form a drop, dissipating it, as they easily do a moist fog, that drop begins to fall

. If it freezes into arising from water. They were indeed rengrain of ice, that ice descends. In descend- dered so faint in passing through it, that when ing, both the drop of water and the grain of collected in the focus of a burning glass, they ice are augmented by particles of the wapour would scarce kindle brown paper. Of course, they pass through in falling, and which they their summer effect in heating the earth was condense by coldness, and attach to them- exceedingly diminished. selves.

Hence the surface was early frozen.. It is possible that, in summer, much of what Hence the first snows remained on it unis rain, when it arrives at the surface of the melted, and received continual additions. earth, might have been snow when it began Hence perhaps the winter of 1783-4, was its descent; but being thawed, in passing more severe than any that had happened for through the warm air near the surface, it is many years. changed from snow into rain.

The cause of this universal fog is not yet How immensely cold must be the original ascertained. Whether it was adventitious to particle of hail, which forms the centre of the this earth, and merely a smoke proceeding future hailstone, since it is capable of commu- from the consumption by fire of some of those nicating sufficient cold, if I may so speak, to great burning balls or globes which we hapfreeze all the mass of vapour condensed round pen to meet with in our rapid course round it, and form a lump of perhaps six or eight the sun, and which are sometimes seen to ounces in weight !

kindle and be destroyed in passing our atmo When, in summer time, the sun is high, sphere, and whose smoke might be attracted and continues long every day above the ho- and retained by our earth; or whether it was rizon, his rays strike the earth more directly the vast quantity of smoke, long continuing and with longer continuance, than in the to issue during the summer from Hecla, in winter; hence the surface is more heated, and Iceland, and that other volcano which arose to a greater depth, by the effect of those rays. out of the sea near that island, which smoke

When rain falls on the heated earth, and might be spread by various winds over the soaks down into it, it carries down with it a northern part of the world, is yet uncertain. great part of the heat, which by that means It seems however worth the inquiry, whe descends still deeper.

ther other hard winters, recorded in history, The mass of earth, to the depth of perhaps were preceded by similar permanent and thirty feet, being thus heated to a certain de- widely extended summer fogs. Because, if gree, continues to retain its heat for some found to be so, men might from such fogs contime. Thus the first snows that fall in the jecture the probability of a succeeding hard beginning of winter, seldom lie long on the winter, and of the damage to be expected by surface, but are soon melted, and soon absorb- the breaking up of frozen rivers in the spring; ed. After which, the winds that blow over and take such measures as are possible and the country on which the snows had fallen, practicable, to secure themselves and effects are not rendered so cold as they would have from the mischiefs that attended the last. been, by those snows, if they had remained, Passy, May, 1784. and thus the approach of the severity of winter is retarded; and the extreme degree of its cold is not always at the time we might ex- To Dr. Lining, at Charleston. pect it, viz. when the sun is at its greatest On Cold produced by Evaporation. * This paper was inserted in the Memoirs of the Li

NEW YORK, April 14, 1757. terary and Philosophical Society of Manchester, Vol. II. It was communicated by Dr. Percival, and of a line from you; and, indeed, the troubles

It is a long time since I had the pleasure read December 22, 1704.

page 378.

of our country, with the hurry of business Igers with one hand, and a piece of wood, of have been engaged in on that account, have the same dimensions, with the other, and bring made me so bad a correspondent, that I both at the same time to the flame of a candle, ought not to expect punctuality in others. you will find yourself obliged to drop the dol

But being about to embark for England, I lar before you drop the wood, because it concould not quit the continent without paying ducts the heat of the candle sooner to your my respects to you, and, at the same time, tesh. Thus, if a silver tea-pot had a handle taking. leave to introduce to your acquaint- of the same metal, it would conduct the heat ance a gentleman of learning and merit, colo- from the water to the hand, and become too nel Henry Bouquet, who does me the favour hot to be used ; we therefore give to a metal to present you this letter, and with whom I am tea-pot a handle of wood, which is not so good sure you will be much pleased.

a conductor as metal. But a china or stone Professor Simpson, of Glasgow, lately com-tea-pot being in some degree of the nature of mnunicated to me some curious experiments glass, which is not a good conductor of heat, of a physician of his acquaintance, by which may have a handle of the same stuff. Thus, it appeared, that an extraordinary degree of also, a damp moist air shall make a man.more cold, even to freezing, might be produced by sensible of cold, or chill him more, than a dry evaporation, I have not had leisure to repeat air that is colder, because a moist air is fitter and examine more than the first and easiest to receive and conduct away the heat of his of viz. Wet the ball of a thermome- body. This fluid, entering bodies in great ter by a feather dipt in spirit of wine, which quantity, first expands them, by separating has been kept in the same room, and has, of their parts a little, afterwards, by farther secourse, the same degree of heat or cold. The parating their parts, it renders solids fluid, mercury sinks presently three or four degrees, and at length dissipates their parts in air. and the quicker, if during the evaporation Take this fluid from melted lead, or from wayou blow on the ball with bellows; a second ter, the parts cohere again, the first grows wetting and blowing, when the mercury is solied, the latter becomes ice: and this is down, carries it yet lower. I think I did not sooner done by the means of good conductors. get it lower than five or six degrees from Thus, if you take, as I have done, a square bar where it naturally stood, which was at that of lead, four inches long, and one inch thick, time sixty. But it is said, that a vessel of together with three pieces of wood planed to water being placed in another somewhat the same dimensions, and lay them on a larger, containing spirit, in such a manner smooth board, fixt so as not to be easily.sepathat the vessel of water is surrounded with rated or moved, and pour into the cavity they the spirit, and both placed under the receiver form, as much melted lead as will fill it, you of an air pump; on exhausting the air, the will see the melted lead chill

, and become spirit, evaporating, leaves such a degree of firm, on the side next the leaden bar, some cold as to freeze the water, though the ther- time before it chills on the other three sides mometer, in the open air, stands inany degrees in contact with the wooden bars, though beabove the freezing point.

fore the lead was poured in, they might all be I know not how this phenomena is to be supposed to have the same degree of heat or accounted for, but it gives me occasion to coldness, as they had been exposed in the mention some loose notions relating to heat same room to the same air. You will likeand cold, which I have for some time enter-wise observe, that the leaden bar, as it has tained, but not yet reduced into any form. cooled the melted lead more than the wooden Allowing common fire, as well as electrical, bars have done, so it is itself more heated by to be a fluid capable of permeating other bo- the melted lead. There is a certain quantidies, and seeking an equilibrium, I imagine ty of this fluid called fire, in every living busome bodies are better fitted by nature to be man body, which fluid, being in due proporconductors of that fluid than others; and, that, tion, keeps the parts of the flesh and blood at generally, those which are the best conduct- such a just distance from each other, as that ors of the electric fluid, are also the best con- the flesh and nerves are supple, and the blood ductors of this; and è contra.

fit for circulation. If part of this due proporThus a body which is a good conductor of tion of fire be conducted away, by means of a fire realily receives it into its substance, and contact with other bodies, as air, water, or conducts it through the whole to all the parts, metals, the parts of our skin and flesh that as metals and water do; and if two bodies, come into such contact first, draw more near both good conductors, one heated, the other together than is agreeable, and give that senin its common state, are brought into contact sation which we call cold; and if too much with each other, the body which has most fire be conveyed away, the body stiffens, the blood readily communicates of it to that which had ceases to flow, and death ensues. On the least, and that which had least readily re- other hand, if too much of this fluid be comceives it, till an equilibrium is produced. municated to the flesh, the parts are separatThus, if you take a dollar between your fin- led too far, and pain ensues, as when they are separated by a pin or lancet. The sensation preparing for distillation, wherein there is a that the separation by fire occasions, we call separation of the spirituous, from the watery heat or burning. My desk on which I now and earthy parts. And it is remarkable, that write, and the lock of my desk, are both ex- the liquor in a distiller's vat, when in its posed to the same temperature of the air, highest and best state of fermentation, as I and have therefore the same degree of heat have been informed, has the same degree of or cold : yet if I lay my hand successively on heat with the human body: that is, about 94 the wood and on the metal, the latter feels or 96. much the coldest, not that it is really so, but Thus, as by a constant supply of fuel in a being a better conductor, it more readily than chimney, you keep a warm room, so, by a the wood takes away and draws into itself the constant supply of food in the stomach, you fire that was in my skin. Accordingly if I keep a warm body; only where little exercise lay one hand, part on the lock, and part on is used, the heat may possibly be conducted the wood, and after it had laid on some time, away too fast; in which case such materials I feel both parts with my other hand, I find are to be used for cloathing and bedding, the part that has been in contact with the lock, against the effects of an immediate contact of very sensibly colder to the touch than the the air, as are, in themselves, bad conductors part that lay on the wood. How a living of heat, and consequently, prevent its being animal obtains its quantity of this Auid called communicated through their substance to the fire, is a curious question. I have shown, air. Hence, what is called warmth in wool, that some bodies (as metals) have a power of and its preference on that account, to linen; attracting it stronger than others; and I have wool not being so good a conductor: and sometimes suspected, that a living body had hence all the natural coverings of animals, to some power of attracting out of the air, or keep them warın, are such as retain and conother bodies, the heat it wanted. Thus me- fine the natural heat in the body, by being tals hammered, or repeatedly bent, grow hot bad conductors, such as wool, hair, feathers, in the beat or hammered part. But when I and the silk by which the silkworm, in its consider that air, in contact with the body, tender embryo state, is first cloathed. Cloathcools it; that the surrounding air is rather ing, thus considered, does not make a man heated by its contact with the body; that warm by giving warmth, but by preventing every breath of cooler air drawn in, carries the too quick dissipation of the heat produced off part of the body's heat when it passes out in his body, and so occasioning an accumuagain; that, therefore there must be in the lation. body a fund for producing it, or otherwise the There is another curious question I will animal would soon grow cold: I have been just venture to touch upon, viz. Whence rather inclined to think, that the fluid fire, as arises the sudden extraordinary degree of well as the fluid air, is attracted by plants in cold, perceptible on mixing some chenrical their growth, and becomes consolidated with liquors, and even on mixing salt and snow, the other materials of which they are formed, where the composition appears colder than the and makes a great part of their substance : coldest of the ingredients ? I have never seen that when they come to be digested, and to the chemical mixtures made, but salt and snow suffer in the vessels a kind of fermentation, I have often mixed myself, and am fully satispart of the fire, as well as part of the air, re- fied that the composition feels much colder to covers its fluid active state again, and diffuses the touch, and lowers the mercury in the itself in the body digesting and separating it: thermometer more thari either ingredient that the fire so reproduced, by digestion and would do separately. I suppose, with others, separation continually leaving the body, its that cold is nothing more than the absence of place is supplied by fresh quantities, arising heat or fire. Now if the quantity of fire befrom the continual separation. That what- fore contained or diffused in the snow and ever quickens the motion of the fluids in an salt was expelled in the uniting of the two animal quickens the separation, and repro- matters, it must be driven away either through duces more of the fire; as exercise. That all the air or the vessel containing them. If it the fire emitted by wood, and other combusti-. is driven off through the air, it must warm bles, when burning existed in them before, in the air, and a thermometer held over the a solid state, being only discovered when se- mixture, without touching it, would discover parating. That some fossils, as sulphur, sea the heat, by the rising of the mercury, as it coal, &c. contain a great deal of solid fire; must, and always does in warm air. and that, in short, what escapes and is dissi- This, indeed, I have not tried, but I should pated in the burning of bodies, besides water guess it would rather be driven off through and earth, is generally the air and fire that the vessel, especially if the vessel be metal, before made parts of the solid. Thus I ima- as being a better conductor than air; and so gine that animal heat arises by or from a kind one should find the bason warmer after such of fermentation in the juices of the body, in mixture. But, on the contrary, the vesse) the same manner as heat arises in the liquors grows cold, and even water, in which the vessel is sometimes placed for the experiment, lected and condensed by the coldness of the freezes into hard ice on the bason. Now I ball, from the moisture in the air, or from our know not how to account for this, otherwise breath; or whether the feather, when dipped than by supposing, that the composition is a into the ether, might not sometimes go through better conductor of fire than the ingredients it

, and bring up some of the water that was separately, and, like the lock compared with under it, I am not certain; perhaps all might the wood, has a stronger power of attracting contribute. The ice continued increasing till fire, and does accordingly attract it suddenly we ended the experiment, when it appeared from the fingers, or a thermometer put into near a quarter of an inch thick all over the it, from the bason that contains it, and from ball, with a number of small spicula, pointing the water in contact with the outside of the outwards. From this experiment one may bason ; so that the fingers have the sensation see the possibility of freezing a man to death of extreme cold, by being deprived of much of on a warm summer's day, it he were to stand their natural fire; the thermometer sinks, in a passage through which the wind blew by having part of itą fire drawn out of the briskly, and to be wet frequently with ether, mercury; the bason grows colder to the touch, a spirit that is more inflammable than brandy as, by having its fire drawn into the mixture, or common spirits of wine. it is become more capable of drawing and It is but within these few years, that the receiving it from the hand; and through the European philosophers seem to have known bason, the water loses its fire that kept its fluid ; this power in nature, of cooling bodies by so it becomes ice. One would expect, that evaporation. But in the east they have long from all this attracted acquisition of fire to been acquainted with it. A friend tells me, the composition, it should become warmer; there is a passage i Bernier's Travels through and, in fact, the snow and salt dissolve at the Hindostan, written near one hundred years same time into water, without freezing. ago, that mentions it as a practice in travel

B. FRANKLIN. ling over dry deserts in that hot climate) to

carry water in flasks wrapt in wet woollen

cloths, and hung on the shady side of the caTo Dr. Lining, at Charleston. mel, or carriage, but in the free air ; whereby, On the production of Cold by Eraporation,

as the cloths gradually grow drier, the water

contained in the flasks is made cool. They LONDON, June 17, 1758. have likewise a kind of earthen pots, unglazIn a former letter I mentioned the experi-ed, which let the water gradually and slowly ment for cooling bodies by evaporation, and ooze through their pores, so as to keep the that I had, by repeatedly wetting the thermo- outside a little wet, notwithstanding the con meter with common spirits, brought the mer- tinual evaporation, which gives great coldcury down five or six degrees. Being lately ness to the vessel, and the water contained at Čambridge, and mentioning this in con- in it. Even our common sailors seem to have versation with Dr. Hadley, professor of che- had some notion of this property; for I remistry there, he proposed repeating the ex- member, that being at sea, when I was a periments with ether, instead of common youth, I observed one of the sailors, during a spirits, as the ether is much quicker in evapo-calm in the night, often wetting his finger in ration. We accordingly went to his chamber, his mouth, and then holding it up in the air, where he had both ether and a thermometer. to discover, as he said, if the air had any moBy dipping first the ball of the thermometer tion, and from which side it came; and this into the ether, it appeared that the ether was he expected to do, by finding one side of his precisely of the same temperament with the finger grow suc!denly cold, and from that side thermometer, which stood then at 65; for it he should look for the next wind; which I then made no alteration in the height of the little laughed at as a fancy. column of mercury. But when the thermo- May not several phenomena, hitherto unmeter was taken out of the ether, and the considered, or unaccounted for, be explained ether, with which the ball was wet, began to by this property ? During the hot Sunday at evaporate, the mercury sunk several degrees. Philadelphia, in June, 1750, when the ther. The wetting was then repeated by a feather mometer was up at 100 in the shade, I sat that had been dipped into the ether, when the in my chamber without exercise, only reading mercury sunk still lower. We continued this or writing, with no other clothes on than a operation, one of us wetting the ball, and an- shirt, and a pair of long linen drawers, the other of the company blowing on it with the windows all open, and a brisk wind blowing bellows, to quicken the evaporation, the mer- through the house, the sweat ran off the backs cury sinking all the time, till it came down of my hands, and my shirt was often so wet, to 7, which is 25 degrees below the freezing as to induce me to call for dry ones to put on; point, when we left off. Soon after it passed in this situation, one might have expected, the freezing point, a thin coat of ice began to that the natural heat of the body 96, added to cover the ball. Whether this was water col- | the heat of the air 100, should jointly have VOL. II. ... 2 Y


created or produced a much greater degree, fore checked and retarded, till we drive away of heat in the body; but the fact was, that my that atmosphere, and bring drier air in its body never grew so hot as the air that sur place, that will receive the vapour, and thererounded it, or the inanimate bodies immersed by facilitate and increase the evaporation ? in the same air. For I remember well, that Certain it is, that mere blowing of air on a the desk, when I laid my arm upon it; a chair, dry body does not cool it, as any one may sawhen I sat down in it; and a dry shirt out of tisfy himself, by blowing with a bellows on the drawer, when I put it on, all felt exceed- the dry ball of a thermoineter; the mercury ing warm to me, as if they had been warmed will not fall; if it moves at all, it rather rises, before a fire. And I suppose' a dead body as being warmed by the friction of the air on would have acquired the temperature of the its surface? To these queries of imagination, air, though a living one, by continual sweat- I will only add one practical observation; that ing, and by the evaporation of that sweat, was wherever it is thought proper to give ease, in kept cold. May not this be a reason why our cases of painful inflammation in the flesh (as reapers in Pennsylvania, working in the open from burnings, or the like) by cooling the field, in the clear hot sun-shine common in our part ; linen cloths, wet with spirit, and applied harvest-time, find themselves well able to go to the part inflamed, will produce the coolness through that labour, without being much in- required, better than if wet with water, and commoded by the heat, while they continue will continue it longer. For water, though to sweat, and while they supply matter for cold when first applied, will soon acquire keeping up that sweat, by drinking frequently warmth from the flesh, as it does not evapoof a thin evaporable liquor, water mixed with rate fast enough; but the cloths wet with rum ; but if the sweat stops, they drop, and spirit, will continue cold as long as any spirit sometimes die suddenly, if a sweating is not is left to keep up the evaporation, the parts again brought on by drinking that liquor, or, warmed escaping as soon as they are warmas some rather choose in that case, a kind of ed, and carrying off the heat with them. hot punch, made with water, mixed with

B. FRANKLIN. honey, and a considerable proportion of vinegar? May there not be in negroes a quicker

J. Bowdoin, in Boston, to Dr. Franklin. evaporation of the perspirable matter from their skins and lungs, which, by cooling them Concerning the Light in Sea Water.— Read at more, enables them to bear the sun's heat the Royal Society, L'ecember 6, 1756. better than whites do? (if that is a fact, as it

November 12, 1753. is said to be ; for the alleged necessity of hav- When I was at the eastward, I had an ing negroes rather than whites, to work in opportunity of observing the luminous appearthe West-India fields, is founded upon it) ance of the sea when disturbed : at the head though the colour of their skins would other and stern of the vessel, when under way, it wise make them more sensible of the sun's appeared very bright. The best opportunity heat, since black cloth heats much sooner, and I had to observe it was in a boat, in company more, in the sun, than white cloth.

I ain per- with several gentlemen going from Portssuaded, from several instances happening month, about three miles, to our vessel lying within my knowledge, that they do not bear at the mouth of Piscataqua river. Soon after cold weather so well as the whites; they will we set off'(it being in the evening) we observed perish when exposed to a less degree of it, a luminous appearance, where the oars dashed and are more apt to have their liinbs frost- the water. Sometimes it was very bright, bitten; and may not this be from the same and afterwards, as we rowed along, gradually cause? Would not the earth grow much hot- lessened, till almost imperceptible, and then ter under the summer-sun, if a constant eva- reillumined. This we took notice of several poration from its surface, greater as the sun times in the passage. When I got on board shines stronger, did not, by tending to cool it, the vessel, I ordered a pail to be dipped up, balance, in some degree, the warmer effects full of sea-water, in which, on the water's of the sun's rays? Is it not owing to the con- being moved a sparkling light appeared. I stant evaporation from the surface of every took a linen cloth, and strained some of the leaf, that trees, though shone on by the sun, water through it, and there was a like appearare always, even the leaves themselves, cool ance on the cloth, which soon went off; but to our sense ? at least much cooler than they on rubbing the cloth with my finger, it was would otherwise be ? May it not be owing renewed. I then carried the cloth to the to this, that fanning ourselves when warm, light, but could not perceive any thing upon does really cool us, though the air is itself it which should cause that appearance. warm that we drive with the fan upon our Several gentlemen were of opinion, that the faces; for the atmosphere round, and next to separated particles of putrid, animal, and other our bodies, having imbibed as much of the bodies, floating on the surface of the sea, might perspired vapour as it can well contain, re-cause that appearance; for putrid fish, &c. ceives no more, and the evaporation is there- they said, will cause it: and the sea-animals

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