FROM JOHN BARTRAM TO B. FRANKLIN.

Relating to a remarkable Aurora Borealis.

READ AT THE ROYAL SOCIETYy, february 25тн, 1762.

Philadelphia, 12 November, 1757. HERE is a visible Aurora Borealis ; at seven o'clock, it was about two hours high, to the northward pretty bright. Soon after daylight disappeared, it was much more east, where it was redder, with some faint streamers, whose points reached near forty-five degrees elevation, which soon disappeared, and the light descended, by degrees, under the pole, and by ten o'clock was near extinct. I should be very glad to know, whether (and how) it appeared this night with you, which may assist in some philosophical inquiries. I have not observed any, this fall, before. Yesterday the weather was cool and clear, and the wind pretty strong at north; and I believe this the coldest evening we have had this fall, though this week we had ice as thick as a dollar.

Extract of the Answer to the above Letter.

London, 11 January, 1758. I THANK you for your account of the Aurora. A very considerable one appeared here the same evening, being Saturday, November 12th. I did not see it, but have heard of it from several. If it was the same, that you saw, it must have been very high, or very extensive, as the two places are one thousand leagues asunder.

SIR,

TO JOHN PRINGLE.

On the different Strata of the Earth.

Craven Street, 6 January, 1758.

I return you Mr. Mitchell's paper on the strata of the earth with thanks. The reading of it, and perusal of the draft that accompanies it, have reconciled me to those convulsions which all naturalists agree this globe has suffered. Had the different strata of clay, gravel, marble, coals, limestone, sand, minerals, &c., continued to lie level, one under the other, as they may be supposed to have done before those convulsions, we should have had the use only of a few of the uppermost of the strata, the others lying too deep and too difficult to be come at; but, the shell of the earth being broke, and the fragments thrown into this oblique position, the disjointed ends of a great number of strata of different kinds are brought up to day, and a great variety of useful materials put into our power, which would otherwise have remained eternally concealed from us. So that what has been usually looked upon as a ruin suffered by this part of the universe, was, in reality, only a preparation, or means of rendering the earth more fit for use, more capable of being to mankind a convenient and comfortable habitation.

I am, Sir, with great esteem, yours, &c.

B. FRANKLIN.

* See this paper, afterwards printed in the Philosophical Transactions.

TO JOHN LINING, AT CHARLESTON.

Experiment with Ether for producing Cold by Evaporation. —Water kept cool in hot Countries. — The human Body cooled by Evaporation.-The Surface of the Earth and Vegetation cooled in the same Manner.

DEAR SIR,

London, 17 June, 1758.

In a former letter I mentioned the experiment for cooling bodies by evaporation, and that I had, by repeatedly wetting the thermometer with common spirits, brought the mercury down five or six degrees. Being lately at Cambridge, and mentioning this in conversation with Dr. Hadley, professor of chemistry there, he proposed repeating the experiments with ether, instead of common spirits, as the ether is much quicker in evaporation. We accordingly went to his chamber, where he had both ether and a thermometer. By dipping first the ball of the thermometer into the ether, it appeared that the ether was precisely of the same temperament with the thermometer, which stood then at 65; for it made no alteration in the height of the little column of mercury. But, when the thermometer was taken out of the ether, and the ether, with which the ball was wet, began to evaporate, the mercury sunk several degrees. The wetting was then repeated by a feather that had been dipped into the ether, when the mercury sunk still lower.

We continued this operation, one of us wetting the ball, and 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 25 '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 Indostan, 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 wrapped 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, 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 exceeding 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 them

* Pennsylvania is in about lat. 40, and the sun, of course, about 12