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another of the company blowing on it with the bellows, to quicken the evaporation, the mercury sinking all the time, till it came down to 7, which is twenty-five degrees below the freezing point, when we left off. Soon after it passed the freezing point, a thin coat of ice began to cover the ball. Whether this was water collected and condensed by the coldness of the ball, from the moisture in the air, or from our breath; or whether the feather, when dipped into the ether, might not sometimes go through it, and bring up some of the water that was under it, I am not certain; perhaps all might contribute. The ice continued increasing till we ended the experiment, when it appeared near a quarter of an inch thick all over the ball, with a number of small spicula, pointing outwards.

From this experiment one may see the possibility of freezing a man to death on a warm summer's day, if he were to stand in a passage through which the wind blew briskly, and to be wet frequently with ether, a spirit that is more inflammable than brandy or common spirits of wine.

It is but within these few years that the European philosophers seem to have known this power in nature of cooling bodies by evaporation. But in the East they have long been acquainted with it. A friend tells me there is a passage in Bernier's Travels through Hindostan, written near one hundred years ago, that mentions it as a practice (in travelling 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 camel, or carriage, but in the free air; whereby, as the cloths gradually grow drier, the water contained in the flasks is made cool. They have likewise a kind of earthen pots, unglazed, which let the water gradually and slowly ooze through their pores, so as to keep the outside a little wet, notwithstanding the continual evaporation, which gives great coldness to the vessel, and the water contained in it. Even our common sailors seem to have had some notion of this property; for I remember that, being at sea when I was a youth, I observed one of the sailors, during a calm in the night, often wetting his finger in his mouth, and then holding it up in the air, to discover, as he said, if the air had any motion, and from which side it came; and this he expected to do, by finding one side of his finger grow suddenly cold, and from that side he should look for the next wind; which I then laughed at, as a fancy.

May not several phenomena, hitherto unconsidered or unaccounted for, be explained by this property? During the hot Sunday at Philadelphia, in June 1750, when the thermometer

was up at 100 in the shade, and I sat in my chamber without exercise, only reading or writing, with no other clothes on than a shirt, and a pair of long linen drawers, the windows all open, and a brisk wind blowing through the house, the sweat ran off the backs of my hands, and my shirt was often so wet as to induce me to call for dry ones to put on. In this situation, one might have expected that the natural heat of the body (96), added to the heat of the air (100), should jointly have created or produced a much greater degree of heat in the body; but the fact was, that my body never grew so hot as the air that surrounded it, or the inanimate bodies immersed in the same air. For I remember well that the desk, when I laid my arm upon it; a chair, when I sat down in it; and a dry shirt out of the drawer, when I put it on, all felt exceedingly warm to me, as if they had been warmed before a fire. And I suppose a dead body would have acquired the temperature of the air, though a living one, by continual sweating, and by the evaporation of that sweat, was kept cold.

May not this be a reason why our reapers in Pennsylvania, working in the open field, in the clear hot sunshine common in our harvest-time, find themselves well able to go through that labor, without being much incommoded by the heat, while they continue to sweat, and while they supply matter for keeping up that sweat by drinking frequently of a thin evaporable liquor, water mixed with rum; but, if the sweat stops, they drop, and sometimes die suddenly, if a sweating is not again brought on by drinking that liquor, or, as some rather choose in that case, a kind of hot punch, made with water, mixed with honey, and a considerable proportion of vinegar? May there not be in negroes a quicker evaporation of the perspirable matter from their skins and lungs, which, by cooling them more, enables them to bear the sun's heat better than whites do? (if this is a fact, as it is said to be; for the alleged necessity of having negroes, rather than whites, to work in the West India fields, is founded upon it) though the color of their skins would otherwise make them more sensible of the sun's heat, since black cloth heats much sooner, and more, in the sun, than white cloth. I am persuaded, from several instances happening within my knowledge, that they do not bear cold weather so well as the whites; they will perish when exposed to a less degree of it, and are more apt to have their limbs frost-bitten; and may not this be from the same cause? Would not the earth much hotter grow under the summer sun, if a constant evaporation from its surface, greater as the sun shines stronger, did not, by tending to

cool it, balance in some degree the warmer effects of the sun's rays? Is it not owing to the constant evaporation from the surface of every leaf, that trees, though shone on by the sun, are always, even the leaves themselves, cool to our sense? at least, much cooler than they would otherwise be? May it not be owing to this that fanning ourselves when warm does really cool us, though the air is itself warm that we drive with the fan upon our faces; for the atmosphere round, and next to our bodies, having imbibed as much of the perspired vapor as it can well contain, receives no more, and the evaporation is therefore checked and retarded, till we drive away that atmosphere, and bring drier air in its place, that will receive the vapor, and therefore facilitate and increase the evaporation? Certain it is, that mere blowing of air on a dry body does not cool it, as any one may satisfy himself, by blowing with a bellows on the dry ball of a thermometer;- the mercury will not fall; if it moves at all, it rather rises, as being warmed by the friction of the air on its surface.

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To these queries of imagination, I will only add one practical observation that, wherever it is thought proper to give ease, in cases of painful inflammation of the flesh (as from burnings, or the like), by cooling the part, linen cloths, wet with spirit, and applied to the part inflamed, will produce the coolness required better than if wet with water, and will continue it longer. For water, though cold when first applied, will soon acquire warmth from the flesh, as it does not evaporate fast enough; but the cloths wet with spirit will continue cold as long as any spirit is left to keep up the evaporation, the parts warmed escaping as soon as they are warmed, and carrying off the heat with them.

[TO MISS STEVENSON.]

SALT WATER RENDERED FRESH BY DISTILLATION
METHOD OF RELIEVING THIRST BY SEA-WATER.

CRAVEN-STREET, August 10, 1761.

WE are to set out this week for Holland, where we may possibly spend a month, but purpose to be at home again before the coronation. I could not go without taking leave of you by a line at least, when I am so many letters in your debt.

In yours of May 19, which I have before me, you speak of the ease with which salt water may be made fresh by distilla

tion, supposing it to be, as I had said, that in evaporation the air would take up water, but not the salt that was mixed with it. It is true that distilled sea-water will not be salt, but there are other disagreeable qualities that rise with the water in distillation; which indeed several besides Dr. Hales have endeavored by some means to prevent, but as yet their methods have not been brought much into use.

I have a singular opinion on this subject, which I will venture to communicate to you, though I doubt you will rank it among my whims. It is certain that the skin has imbibing as well as discharging pores; witness the effects of a blistering-plaster, &c. I have read that a man, hired by a physician to stand by way of experiment in the open air naked during a moist night, weighed near three pounds heavier in the morning.

I have often observed myself, that, however thirsty I may have been before going into the water to swim, I am never long so in the water. These imbibing pores, however, are very fine,perhaps fine enough in filtering to separate salt from water; for though I have soaked (by swimming, when a boy) several hours in the day for several days successively in salt water, I never found my blood and juices salted by that means, so as to make me thirsty or feel a salt taste in my mouth; and it is remarkable that the flesh of sea-fish, though bred in salt water, is not salt.

Hence I imagine, that if people at sea, distressed by thirst, when their fresh water is unfortunately spent, would make bathing-tubs of their empty water-casks, and, filling them with sea-water, sit in them an hour or two each day, they might be greatly relieved. Perhaps keeping their clothes constantly wet might have an almost equal effect, and this without danger of catching cold. Men do not catch cold by wet clothes at sea. Damp but not wet linen may possibly give colds, but no one catches cold by bathing, and no clothes can be wetter than water itself. Why damp clothes should then occasion colds, is a curious question, the discussion of which I reserve for a future letter, or some future conversation.

[TO MISS STEVENSON.]

TENDENCY OF RIVERS TO THE SEA EFFECTS OF THE SUN'S RAYS ON CLOTHS OF DIFFERENT COLORS.

September 20, 1761.

MY DEAR FRIEND: It is, as you observed in our late conversation, a very general opinion that all rivers run into the sea, or deposit their waters there. 'Tis a kind of audacity to call such general opinions in question, and may subject one to censure. But we must hazard something in what we think the cause of truth; and, if we propose our objections modestly, we shall, though mistaken, deserve a censure less severe than when we are both mistaken and insolent.

That some rivers run into the sea is beyond a doubt: such, for instance, are the Amazon, and I think the Oronoco and the Mississippi. The proof is, that their waters are fresh quite to the sea, and out to some distance from the land. Our question is, whether the fresh waters of those rivers whose beds are filled with salt water to a considerable distance up from the sea (as the Thames, the Delaware, and the rivers that communicate with Chesapeake Bay in Virginia) do ever arrive at the sea? And, as I suspect they do not, I am now to acquaint you with my reasons; or, if they are not allowed to be reasons, my conceptions at least, of this matter.

The

The common supply of rivers is from springs, which draw their origin from rain that has soaked into the earth. union of a number of springs forms a river. The waters, as they run, exposed to the sun, air and wind, are continually evaporating. Hence in travelling one may often see where a river runs, by a long bluish mist over it, though we are at such a distance as not to see the river itself. The quantity of this evaporation is greater or less, in proportion to the surface exposed by the same quantity of water to those causes of evaporation. While the river runs in a narrow, confined channel in the upper, hilly country, only a small surface is exposed; a greater, as the river widens. Now, if a river ends in a lake, as some do, whereby its waters are spread so wide as that the evaporation is equal to the sum of all its springs, that lake will never overflow: - and if, instead of ending in a lake, it was drawn into greater length as a river, so as to expose a surface equal in the whole to that lake, the evaporation would be equal, and such river would end as a canal; when the ignorant might suppose, as they actually

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