Imatges de pàgina
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fered, however, confiderably in magnitude and brightnefs; for the volcano of the year 1783, though much brighter than that which is now burning, was not nearly fo large in the dimensions of its eruption: the former feen in the telefcope refembled a star of the fourth magnitude as it appears to the natural eye; this, on the contrary, fhews a vifible difk of ki• minous matter, very different from the fparkling brightnefs of ftar light.

P. S. M. Méchain having fa voured me with an account of the difcovery of his coinet, I looked for it among the Pleiades, fuppofing its track fince the 10th of this month to lie that way; and faw it April 19th, at 10 h. 10' fidereal time, when it preceded FL d Pleiadum about 54′′ in time, with nearly the fame declination as that flar; but no great accuracy was attempted in the determination of its place. As I have mentioned the comet in a foregoing paragraph of this paper, I thought it proper here to add my oblervation of it. "The comet is nearly round, with

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a fmall tail towards the north "following part: the chevelure "extends to about four or five

minutes; and it has a central, "very fmall, ill-defined nucleus, "of no great brightness."

EXPERIMENTS on the MÕISTURE abforbed from the ATMO-
SPHERE by various SUBSTANCES.
THOMPSON, Knt. F. R.S.

By Sir BENJAMIN

[From the fame Work.]

EING engaged in a course of with respect to heat, and particu

"B con- as ducting powers of various bodies monly made ufe of for cloathing,

in order to fee if I could difcover any relation between the conducting powers of thofe fubitances, and their power of abforbing moisture from the atmosphere, I made the following experiments.

"Having provided a quantity of each of the undermentioned fubitances, in a state of the most perfect cleanness and purity, I expofed them, fpread out upon clean Chinaplates, twenty-four hours in the dry air of a very warm room (which had been heated every day for feveral months by a German stove), the last fix hours the heat being kept up to 85° of Fahrenheit's thermometer; after which I entered the room with a very accurate balance, and weighed equal quantities of these various fubftances, as expreffed in the following table.

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hours, upon a table placed in the middle of a room, the air of the room being at the temperature of 45° F.; atter which they were carefully weighed (in the room and were found to weigh as under mentioned.

Linen Ravelings of fine linen Cotton wool Silver wire, very fine, gilt, and flatted, being the ravelings of gold lace.

"They were then removed into a very damp cellar, and placed upon a table, in the middle of a vault, where the air, which appeared by the hygrometer to be completely faturated with moiffure, was at the temperature of 45° F.; and in this fituation they were fuffered to remain three days and three nights, the vault being hung round, during all this time, with wet linen cloths, to render the air as damp as puttible, and the door of the vault being

fhut.

Weight after being dried 24 hours in a hot

room.

"At the end of the three days I entered the vault, with the balance, and weighed the various fubftances upon the pot, when they were found to weigh as is expreffed in the third column of the following table.

Pts.

1000

1000

1000

1000

1000

1000

IOCO

1000

1000

1000

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N. B. The weight made ufe of in these experiments was that of Cologne, the parts or leaft divifions being part of a mark, confequently 1000 of thefe parts make about 524 grains of Troy..

"I did not add the filver wire to the bodies above mentioned from any idea that that fubflance could poffibly imbibe moisture from the atmosphere; but I was willing to fee whether a metal, placed in air faturated with water, is not capable of receiving a fmall addition of weight from the moisture attracted by it, and attached to its furface; from the refult of the experiment, however, it should feem that no fuch attraction fubfifts between the metal I made ufe of, and the watery vapour diffolved in air.

"I was totally mistaken in my conjectures relative to the refults of the experiments with the other fubftances. As linen is known to attract water with so much avidity; and as, on the contrary, wool, bair, feathers, and other like animal fubstances, are made wet with fo much difficulty, I had little doubt but that linen would be found to attract moisture from the atmosphere with much greater force than any of thofe fubftances; and that, under fimilar circumstances, it would be found to contain much more water and I was much confirmed in this opinion upon recollecting the great difference in the apparent dampoefs of linen and of woollen clothes, when they are both expofed to the fame atmosphere. But thefe experiments have convinced me, that all my fpeculations were founded upon erroneous principles.

"It should feem, that thofe bodies which are the most eafily wet, or which receive water, in its unelaftic form. with the greatest eale, are not thofe which in all cafes at

tract the watery vapour diffolved in the air with the greatest force.

"Perhaps the apparent dampness of linen, to the touch, arifes inore from the ease with which that subftance partswith the water it contains, than from the quantity of water it actually holds: in the fame manner as a body appears hot to the touch, in confequence of its parting freely with its heat, while ano her body, which is actually at the fame temperature, but which witholds its heat with greater obftinacy, affects the fenfe of feeling much less violently.

"It is well known, that woollen clothes, fuch as flannels, &c. worn next the fkin, greatly promote infenfible perfpiration. May not this arife principally from the ftrong attraction which fubfits between wool and the watery vapour which is continually itluing from the hu man body?

"That it does not depend entirely upon the warmth of that covering, is clear; for the fame degree of warmth, produced by wearing more clothing of a different kind, does not produce the fame effect.

"The perfpiration of the human body being abforbed by a covering of flannel, it is immediately ditiributed through the whole thickness of that fubstance, and by that means expofed by a very large furface to be carried off by the atmosphere; and the loss of this watery vapour, which the flannel fuftains on the one fide, by evaporation, being immediately restored from the other, in confequence of the frong attrac tion between the flannel and this vapour, the pores of the fkin are. difencumbered, and they are continually furrounded by a dry, warp, and falubrious atmosphere.

"I am astonished, that the cuf tom of wearing flannel next the fkin fhould

Should not have prevailed more uni. vertally. I am confident it would prevent a multitude of difeafes; and I know of no greater luxury than the comfortable fenfation which arifes from wearing it, efpecially after one is a little accustomed

to it.

"It is a mistaken notion, that it is too warm a cloathing for fummer. I have worn it in the hottest climates, and in all feafons of the year, and never found the leaft inconvenience from it. It is the warm bath of a perfpiration confined by a linen fhirt, wet with fweat, which renders the fummer heats of fouthern climates fo infupportable; but flannel promotes perfpiration, and favours its evaporation; and evaporation, as is well known, produces pofitive cold.

"I first began to wear flannel, not from any knowledge which I had of its properties, but merely upon the recommendation of a very able phyfician (fir Richard Jebb); and when I began the experiments of which I have here given an account, I little thought of dif covering the phyfical caufe of the good effects which I had experienced from it; nor had I the most distant idea of mentioning the circumftance. I fhall be happy, however, if what

I have faid, or done, upon the fubject, fhould induce others to make a trial of what I have fo long experienced with the greate advantage, and which, I am confident, they will find to contribute greatly to health, and confequently to all the other comforts and enjoyments of life.

"I fhall then think thefe expe. riments, trifling as they may appear, by far the molt fortunate, and the most important ones I have ever made.

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"MY

Y journey to the northern mountains in January last, in attendance upon the vizier, gave me an opportunity of fatisfying, in fome degree, my curiofity on the fubject you are fo defirous of being

1787.

With regard to the original object of thefe experiments, the difcovery of the relation which I thought might poffibly fublitt between the warmth of the fubftances in question, when made ufe of as cloathing, and their powers of attracting moisture from the atmofphere; or, in other words, between the quantities of water they contain, and their conducting pow ers with regard to heat; I could not find that thefe properties depended in any manner upon, or were in any way connected with, each other.

"The refult of my experiments upon the conducting powers of thefe fubftances, I referve for a future communication."

On the PRODUCTION of BORAX. In a Letter from WILLIAM

BLAZE, Efq.

[From the fame Work.]

informed of, the production and manufacture of borax. The place which his excellency vifited is called Betowle, and is a small principality in the first of the northern mountains, where they rife from the plains

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plains of Hindoftan, and is diftant from Lucknow about two hundred miles N. E. The town is a principal mart, where the commoditics of the mountains are exchanged for thofe of the plain. The raja, or prince of the country, holds his poffeffions in the hills as an independent fovereign; but for thofe on the plain he owes fealty, and pays tribute to the vizier. He therefore embraced this opportunity of paying homage in perfon to his lord. During his ftay at court, I had an opportunity of making the enquiries I withed from his people, and particularly from his dewan or minifter, who had with him fome of the inhabitants of the place where the borax is made.

"This faline fubftance, called in the language of this country favagab, is brought into Hindoftan from the mountains of Tibbet. The place where it is produced is in the kingdom of Jumlate, diftant from Betowle about thirty days journey north. Jumlate is the largest of the kingdoms in that part of the Tibbet mountains, and is confidered as holding a fuperiority over all the reft.

"The place where the borax is produced is defcribed to be in a finall valley, furrounded with fnowy mountains, in which is a lake, about fix miles in circumference, the water of which is conftantly hot, fo much fo that the hand cannot be held in it for any time. The ground round the banks of the lake is perfectly barren, not producing even a blade of grifs; and the earth is full of a faline matter in fuch plenty that, after falls of rain or fnow, it concretes in white flakes upon the furface, like the natron in Hindoftan. Upon the banks of this lake, in the winter featon, when the falls of how begin, the earth is formed

into fmall refervoirs, by railing it into banks about fix inches high; when thefe are filled with fnow, the hot water from the lake is thrown upon it, which, together with the water from the melted frow, remains in the refervoir, to be partly abforbed by the earth, and partly evaporated by the fun; atter which there remains at the bottom a cake of fometimes half an inch thick, of crude borax, which is taken up and referved for ufe. It can only be made in the winter feafon, becaufe the falls of fnow are indifpenfably requifite, and alfo because the faline appearances upon the earth are strongest at that season. When once it has been made upon any spot, in the manner above defcribed, it cannot be made again upon the fame place, till the fnow fhall have fallen upon it and diffolved three or four times; after which the faline efflorefcence reappears, and it is again fit for the operation.

"The borax in the fate above defcribed, is tranfported from hill to hill upon goats, and paffes through many different hands before it reaches the plains, which increases the difficulty of obtaining authentic information regarding the original manufacture. When brought down from the hills, it is refined from the earth and grofs impurities by boil ing and cryftallifation. I could obtain no anfwers to any question regarding the quality of the water, and the mineral productions of the foil. All they could fay of the former was, that it was very hot, very foul, and as it were greafy; that it boils up in many places, and has a very offenfive fmell: and the latter remarkable only for the faline appearances above described. That country, however, in general, produces confiderable quantities of iron,

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