A very different refult attended the Author's next experiment-apparently in confequence only of his having introduced into the receiver, through an aperture in the top of it, a piece of the oiled leather abovementioned. On working the pump ten minutes as before, the 'barometer gage marked a degree of exhaustion equal only to 300; but the pear gage indicated a degree of rarefaction not less than 4000.

On removing the oiled leather, and again working the pump, the two gages agreed perfectly, as before.

That he might, as it were, analyse the leather, or discover to what principle contained in it this aftonifhing variety was owing; the Author feparately and fucceffively included in the receiver two ounces of tallow; the fame quantity of oil; and of allum; and a piece of leather in the ftate in which it is received from the leather-fellers. With the three first of these fubftances included in the receiver the exhaustion did not exceed 600, as indicated by either of the gages: but when the leather alone was put into it, the difference in the teftimony of the two gages was very remarkable. The elastic vapour fuppofed to proceed from it fupplied the place of the exhaufted air fo very faft, that the barometer gage, after ten minutes working of the pump, could not be brought to indicate a degree of exhauftion greater than 159; whereas the pear gage indicated a degree of rarefaction that was estimated at one hundred thoufand.

To determine whether this enormous variation in the testimonies of the two gages proceeded from moisture in the leather, the Author repeated the experiment with a piece of fresh leather that weighed 100 grains. The pear gage, as before, indicated a rarefaction of 100,000. The fame piece of leather being tried again, after it had been dried at the fire till it would lofe no more of its weight, the pear gage exhibited a rarefaction of only 280. The leather was next held over the fteam of hot water till it had recovered its former weight and moisture; and again the degree of exhauftion by the pear gage appeared to be 100,000. In all these three trials, the degree of exhauftion by the barometer, gage never exceeded 268.

Among the various fubftances afterwards included in the receiver, the four following likewife produced a degree of rarefaction estimated at 100,000, as indicated by the pear gage; viz. a piece of the infide of a China orange; fome of the infide of an onion; a piece of tainted beef, and a piece of fresh beef: each of thefe weighed 100 grains, and loft about two grains during the experiment. In none of thefe trials did the barometer gage indicate a greater degree of rarefaction than 160.

When

When oil of vitriol was put under the receiver, a very fingu lar effect, and indeed contrary to thofe above mentioned, was produced the pear gage then conftantly indicating a much fmaller degree of rarefaction than the barometer gage. Part of this effect might poffibly be owing to the vitriolic acid's attracting and condenfing the aqueous vapour fufpended in the air contained in the receiver. The acid acquired a small additional weight during each trial; part of which, however, it might collect from the open air, during the time spent in weighing it.

From another fet of the Author's experiments, it appears, that when the receiver is placed, as is very ufual, on leather foaked in water, or in fpirit of wine and water, the pump is prevented from exhaufting to any confiderable degree; that is, according to the teftimony of the barometer gage.-In a prelimi nary trial, when the receiver was placed on the dry pump-plate, with only a little oil poured round the outfide edge of it, the barometer gage and pear gage agreed in indicating a rarefaction of 600 as before: but when the receiver was fet on wet leather, the rarefaction, in fix different trials, never exceeded 51. In thefe experiments it is obfervable that the degree of exhauftion, as indicated by the pear gage, varied, somewhat unaccountably, from 500 to 16,000.

The bad effects refulting from the ufing of water in the barrel, or from employing it in foftening the leathers of the pistons, are rendered evident by two of the Author's experiments; where the highest degree of rarefaction that could be procured, under thefe circumftances, was 37 according to the barometer gage, and 38 according to the pear gage.

The effects of a vapour on the barometer gage, is in none of thefe experiments more confpicuous than in the 61st and last. In confequence of putting a vial of ather under the receiver, for the purpose of producing artificial cold; though the pump was worked half an hour, the apparent degree of exhauftion, according to the barometer gage, was only 16:—and yet this very pump exhaufted above 400 times, according to the fame gage, before the æther was put under the receiver.

We fhall just mention, by the bye, that, in the experiment preceding this, the Author produced a cold by means of ather, in the exhaufted receiver, which was 48 degrees below o in Fahrenheit's thermometer; that is 103 degrees below 55°. the temperature of the air in the room where the experiment was made.

Thefe are fome of the more material parts of Mr. Nairne's very ingeniou experimental investigation of this curious fubject. We would recommend, however, the perufal of the whole

article,

article, as well as the further profecution of the inquiry itself, to our philofophical readers; especially as, though the general refults may be depended upon, a few anomalies or irregularities occurred in fome of the Author's trials, the caufes of which may be detected by further experiments *.

Article 22. Barometrical Obfervations on the Depth of the Mines

in the Hartz. By John Andrew De Luc, F. R. S.

M. De Luc's ingenious and extenfive refearches and operations relative to the menfuration of heights, by means of the barometer, are well known to the philofophical world +; and, exclufive of the prefent article, form the foundation of the two very interefting papers which fucceed it. In this memoir M. De Luc gives an account of the observations which he made in the very deep mines of the Hartz, in order to difcover whether the formula or rules given by him for the measuring of heights above ground, by the obferved rarefactions of the air, as indicated by the barometer, would anfwer in the measuring of depths under ground: or, in other words, whether the con

That even mercury, the heaviest of all known fluids, is elevated into vapour in the Torricellian vacuum, has been long obferved by the Reviewer of the prefent article. In one barometer particularly, which terminates above in a large ball, he has during more than seven years past seen that part of the furface of the ball which was next to a window near which the inftrument was fufpended, ftudded with numerous mercurial globules, elevated from the furface of the quickfilver in the tube, which was at leaft five inches below it. Some of thefe, when they have acquired a pretty large fize, roll down, and are fucceeded by others. After repeatedly washing them down, by inclining the tube, fresh globules, though at firft vifible only with a magnifier, may be perceived in the space of a day or two, which afterwards increase both in number and fize. Thefe globules never appear except on that fide of the bulb next the window. On turning the tube half round, they gradually become fmaller, and at length difappear; while a fresh fucceffion of globules begin to appear on the oppofite fide, or that which is now next the window.-Colonel Roy, in one of the following articles, takes notice of his having obferved mercury converted into vapour, in vacuo, and condenfed in the upper part of the tube, in fome of his experiments, when it was heated to 100°. F. and upwards. Our obfervation fhews that this evaporation takes place in the common temperature of the atmosphere; and in the winter as well as fummer. Indeed, there are few barometers in which fome mercurial globules may not be perceived, in a void part of the tube, on a near infpection. That an elaftic vapour rifes from heated water, in vacuo, and preffes on the furface of the mercury, in the bafon of the common barometer-gage, was obferved by the Abbé Nollet. See Mem. de l Acad. des Sciences, Année 1748, pag. 122. Edit. in 12mo.

+ See our Review, vol. xlviii. Append. p. 56. Vol. xlix. Append. p. 579, and vol. L. p. 557.

denfations

denfations of the air, in deep pits, which contain exhalations of various kinds, follow the fame laws with the rarefactions obferved by him on the mountains in the neighbourhood of Geneva; where the obfervations were made on which his formula were founded. He gives in this article a detail of his fubterraneous barometrical obfervations, which were found to agree very nearly with the geometrical meafures that had before been taken by the miners.

As fome doubts might be entertained with respect to the accuracy of these last measures, which had been taken by the fubterranean geometer, with scarce any other apparatus than a twisted brass wire five fathoms long, a femi-circle, and a compafs; the Author remarks that thefe obfervations are of too much importance to the miners to be taken in a negligent or inaccurate manner. Daily experience, M. De Luc obferves, evinces the truth and exactness of these fubterranean operations. Fully confiding in the truth of his obfervations, a miner- in the abfolute obfcurity of the entrails of the earth, undertakes a labour that is to coft him years, in daily boring. through a rock. Another miner fets out to meet him, from fame other mine, or from without. At the end of a determined meafure, the gnomes begin to hear each other, and at length they meet. I have obferved fome of these points of rencounter in the galleries; it is fometimes difficult to perceive the small winding which has been neceffary for their meeting end to end.' Article 29. Obfervations made in Savoy, in order to afcertain the Height of Mountains by Means of the Barometer; being an Examination of M. De Luc's Rules, delivered in his Recherches, &c. By Sir George Shuckburg, Bart. F. R. S.

This philofophical traveller, in the course of a tour into Italy, in the years 1775 and 1776, made fome stay at Geneva, and being provided with a large and excellent collection of philofophical inftruments, he had the laudable curiofity, and the perfeverance, to verify or repeat M. De Luc's barometrical experiments on the spot where they were originally made.-From feveral of his obfervations it follows, that fome correction of the barometrical rules given by M. De Luc is neceffary: we mean particularly with refpect to the true ratio between the specific gravities of air and quicksilver; or in the expreffing the value of an inch of quickfilver in the Torricellian tube, in corresponding inches of the atmosphere, the temperature being given.

From the mean of the Author's obfervations, on the Mole, and on Mont Saleve, one of M. De Luc's ftations, it may be inferred that M. De Luc's rules give the difference of elevation too little by about 23 feet in every 1000 feet; and confe quently that the atmosphere is rather lighter than he prefumed it to be. The Author feems inclined to afcribe this error of fo diligent

diligent and accurate an obferver to his not having placed his barometers fufficiently near each other in an horizontal direction; whereas his were never feparated more than two or three miles.

The fecond part of this article confits of precepts, as well as tables, for calculating any acceffible heights or depths. These are so conftructed as to be eafily underflood by perfons who are not converfant with logarithms, or mathematical computations. A table likewife is added of a great number of heights taken by the barometer, at various places in France, Savoy, and Italy..

The conclufion of the Author's defcription of his afcending the Mole-a steep infulated mountain eighteen miles east of Geneva-is fufficient to make the reader giddy. We had now,' fays he, reached the fummit; and there my curiofity finished in aftonishment. I perceived myself elevated 6000 feet in the atmosphere, and standing as it were on a knife edge, for fuch is the figure of the ridge or top of this mountain; length without breadth, or the least appearance of a plain, as I had expected to find. Before me an immediate precipice, à pic, of above 1000 feet, and behind me the very steep afcent I had juft now mounted. I was imprudently the firft of the company: the furprize was perfect horror, and two steps further had fent me headlong from the rock.

Article 34. Experiments and Obfervations made in Britain, in order to obtain a Rule for measuring Heights with the Barometer. By Colonel William Roy, F. R. S.

Thefe obfervations and experiments are intended ftill further to improve M. De Luc's barometrical method of mensuration; but they are too numerous and complicated to admit of any very fatisfactory extract or abridgment. We fhall briefly mention, however, a few of the refults.

In the first fection, the Author inquires into the rate of expanfion of quickfilver by various degrees of heat, as afcertained by means of an apparatus contrived for that purpofe; in which the mercury contained in a barometer tube had various temperatures communicated to it, from the freezing point to that of boiling water. From thefe trials it appears, that the mercury ftanding at 30 inches, in the temperature of 32 F. fuffers, not an equal or uniform, but a progreffive expanfion, on being gradually affected by increafing degrees of heat: the expansions in the lower parts of the fcale being greater than those produced in the higher temperatures. It appears too, that when the above mentioned mercurial column has acquired the heat of boiling water, it is lengthened's parts of an inch.

In the fecond fection, the Author endeavours to afcertain the expansion of air by heat, through the means of the manometer. From this fet of experiments he infers that it is past a REV. June, 1778.

H h

doubt