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not probably acquire their magnitude before they began to descend. The air, being eight hundred times rarer than water, is unable to support it but in the shape of vapor, a state in which its particles are separated. As soon as they are condensed by the cold of the upper region so as to form a drop, that drop begins to fall. If it freezes into a grain of ice, that ice descends. In descending, both the drop of water and the grain of ice are augmented by particles of the vapor they pass through in falling, and which they condense by coldness, and attach to themselves. It is possible that, in summer, much of what is rain when it arrives at the surface of the earth might have been snow when it began its descent; but, being thawed in passing through the warm air near the surface, it is changed from snow into rain. How immensely cold must be the original particle of hail, which forms the centre of the future hailstone, since it is capable of communicating sufficient cold, if I may so speak, to freeze all the mass of vapor condensed round it, and form a lump of perhaps six or eight ounces in weight ! When, in summer time, the sun is high, and continues long every day above the horizon, his rays strike the earth more directly, and with longer continuance, than in the winter; hence the surface is more heated, and to a greater depth, by the effect of those rays. When rain falls on the heated earth, and soaks down into it, it carries down with it a great part of the heat, which by that means descends still deeper. The mass of earth, to the depth of perhaps thirty feet, being thus heated to a certain degree, continues to retain its heat for some time. Thus the first snows that fall in the beginning of winter seldom lie long on the surface, but are soon melted, and soon absorbed. After which, the winds that blow over the country on which the snows had fallen are not rendered so cold as they would have been, by those snows, if they had remained, 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 expect it, namely, when the sun is at its greatest distance, and the day shortest, but some time after that period, according to the English proverb, which says, “as the day lengthens, the cold strengthens; ” the causes of refrigeration continuing to operate, while the sun returns too slowly, and his force continues too weak, to counteract them. During several of the months of the year 1783, when the effects of the sun's rays to heat the earth in these northern regions should have been the greatest, there existed a constant fog over all Europe, and great part of North America. This fog was of a permanent nature : it was dry, and the rays of the sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint, in passing through it, that when collected in the focus of a burning-glass they would scarce kindle brown paper. Of course, their summer effect in heating the earth was exceedingly diminished. Hence the surface was early frozen. Hence the first snows remained on it unmelted, and received continual additions. Hence perhaps the winter of 1783–4 was more severe than any that had happened for many years. The cause of this universal fog is not yet ascertained. Whether it was adventitious to this earth, and merely a smoke proceeding from the consumption by fire of some of those great burning balls or globes which we happen to meet with in our rapid course round the sun, and which are sometimes seen to kindle and be destroyed in passing our atmosphere, and whose smoke might be attracted and retained by our earth; or whether it was the vast quantity of smoke, long continuing to issue during the summer from Hecla, in Iceland, and that other volcano which arose out of the sea near that island, which smoke might be spread by various winds over the northern part of the world, is yet uncertain. It seems, however, worth the inquiry, whether other hard winters, recorded in history, were preceded by similar permanent and widely-extended summer fogs. Because, if found to be so, men might from such fogs conjecture the probability of a succeeding hard winter, and of the damage to be expected by the breaking up of frozen rivers in the spring, and take such measures as are possible and practicable to secure themselves and effects from the mischiefs that attended the last.
[To DR. LINING, AT CHARLESTON.]
ON COLD PRODUCED BY EVAPORATION.
NEw York, April 14, 1757. IT is a long time since I had the pleasure of a line from you; and, indeed, the troubles of our country, with the hurry of business I have been engaged in on that account, have made me so bad a correspondent, that I ought not to expect punctuality in others. But, being about to embark for England, I could not quit the continent without paying my respects to you, and, at the same time, taking leave to introduce to your acquaintance a gentleman of learning and merit, Colonel Henry Bouquet, who does me the favor to present you this letter, and with whom I am sure you will be much pleased. -Professor Simpson, of Glasgow, lately communicated to m some curious experiments of a physician of his acquaintance, by which it appeared that an extraordinary degree of cold, even to freezing, might be produced by evaporation. I have not had leisure to repeat and examine more than the first and easiest of them, namely: Wet the ball of a thermometer by a feather dipped in spirit of wine, which has been kept in the same room, and has, of course, the same degree of heat or cold. The mercury sinks presently three or four degrees, and the quicker if during the evaporation you blow on the ball with bellows; a second wetting and blowing, when the mercury is down, carries it yet lower. I think I did not get it lower than five or six degrees from where it naturally stood, which was at that time sixty. But it is said that, a vessel of water being placed in another somewhat larger, containing spirit, in such a manner that the vessel of water is surrounded with the spirit, and both placed under the receiver of an air-pump, on exhausting the air, the spirit, evaporating, leaves such a degree of cold as to freeze the water, though the thermometer in the open air stands many degrees above the freezing point. I know not how this phenomena is to be accounted for; but it gives me occasion to mention some loose notions relating to heat and cold which I have for some time entertained, but not yet reduced into any form. Allowing common fire, as well as electrical, to be a fluid capable of permeating other bodies, and seeking an equilibrium, I imagine some bodies are better fitted by nature to be conductors of that fluid than others; and that generally those which are the best conductors of the electric fluid are also the best conductors of this; and 3 contra. Thus, a body which is a good conductor of fire readily receives it into its substance, and conducts it through the whole to all the parts, as metals and water do; and if two bodies, both good conductors, one heated and the other in its common state, are brought into contact with each other, the body which has most fire readily communicates of it to that which had least, and that which had least readily receives it, till an equilibrium is produced. Thus, if you take a dollar between your fingers with one hand, and a piece of wood of the same dimensions with the other, and bring both at the same time to the flame of a candle, you will find yourself obliged to drop the dollar before you drop the wood, because it conducts the heat of the candle sooner to your flesh. Thus, if a silver tea-pot had a handle of the same metal, it would conduct the heat from the water to the hand, and become too hot to be used; we therefore give to a metal tea-pot a handle of wood, which is not so good a conductor as metal. But a china or stone tea-pot, being in some degree of the nature of glass, which is not a good conductor of heat, may have a handle of the same stuff. Thus, also, a damp, moist air shall make a man more sensible of cold, or chill him more, than a dry air that is colder; because a moist air is fitter to receive and conduct away the heat of his body. This fluid, entering bodies in great quantity, first expands them by separating their parts a little ; afterwards, by further separating their parts, it renders solids fluid, and at length dissipates their parts in air. Take this fluid from melted lead, or from water, the parts cohere again, – the first grows solid, the latter becomes ice; and this is sooner done by the means of good conductors. Thus, if you take, as I have done, a square bar of lead, four inches long and one inch thick, together with three pieces of wood planed to the same dimensions, and lay them on a smooth board, fixed so as not to be easily separated or moved, and pour into the cavity they form as much melted lead as will fill it, you will see the melted lead chill and become firm on the side next the leaden bar, some time before it chills on the other three sides, in contact with the wooden bars, though before the lead was poured in they might all be supposed to have the same degree of heat and coldness, as they had been exposed in the same room to the same air. You will likewise observe that the leaden bar, as it has cooled the melted lead more than the wooden bars have done, so it is itself more heated by the melted lead. There is a certain quantity of this fluid called fire in every living human body, which fluid, being in due proportion, keeps the parts of the flesh and blood at such a just distance from each other as that the flesh and nerves are supple, and the blood fit for circulation. If part of this due proportion of fire be conducted away, by means of a contact with other bodies, as air, water or metals, the parts of our skin and flesh that come into such contact first draw more near together than is agreeable, and give that sensation which we call cold; and, if too much be conveyed away, the body stiffens, the blood ceases to flow, and death ensues. On the other hand, if too much of this fluid be communicated to the flesh, the parts are separated too far, and pain ensues, as when they are separated by a pin or lancet. The sensation that the separation by fire occasions we call heat or burning. My desk on which I now write, and the lock of my desk, are both exposed to the same temperature of the air, and have therefore the same degree of heat or cold : yet, if I lay my hand successively on the wood and on the metal, the latter feels much the coldest; not that it is really so, but, being a better conductor, it more readily than the wood takes away and draws into itself the fire that was in my skin. Accordingly, if I lay one hand part on the lock and part on the wood, and, after it has laid on some time, I feel both parts with my other hand, I find the part that has been in contact with the lock very sensibly colder to the touch than the part that lay on the wood. How a living animal obtains its quantity of this fluid called fire is a curious question. I have shown that some bodies (as metals) have a power of attracting it stronger than others; and I have sometimes suspected that a living body had some power of attracting out of the air, or other bodies, the heat it wanted. Thus, metals hammered, or repeatedly bent, grow hot in the beat or hammered part. But, when I consider that air in contact with the body cools it; that the surrounding air is rather heated by its contact with the body; that every breath of cooler air drawn in carries off part of the body's heat when it passes out again; that therefore there must be in the body a fund for producing it, or otherwise the animal would soon grow cold, – I have been rather inclined to think that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes consolidated with the other materials of which they are formed, and makes a great part of their substance; that when they come to be digested, and to suffer in the vessels a kind of fermentation, part of the fire, as well as part of the air, recovers its fluid active state again, and diffuses itself in the body, digesting arid separating it; that the fire so reproduced, by digesting and separation, continually leaving the body, its place is supplied by fresh quantities, arising from the continual separation; that whatever quickens the motion of the fluids in an animal quickens the separation, and reproduces more of the fire, as exercise; that all the fire emitted by wood, and other combustibles, when burning, existed in them before, in a solid state, being only discovered when separating; that some fossils, as sulphur, sea-coal, &c., contain a great deal of Solid fire;