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it. And as our philofophical readers may wish to know the real height or extent of the earth's atmosphere, as deduced from a long courfe of experiments, made with the most perfect inftruments, and the refults of which have been calculated with a degree of precifion never perhaps before attained in this inquiry; we fhall, in the first place, give them the substance of his final conclufions on this fubject; premifing only a fhort account of the material principles, or elements, on which they are founded.

M. De Luc parts from this well known poftulatum; that the density of the air is every where proportional to the weight with which it is preffed. M. Bouger however was induced to conteft the truth of this fundamental propofition, on finding that his formula, deduced from numerous obfervations made in the fuperior parts of the atmosphere, and which agreed with actual obfervations made in the upper parts of the cordeliers, gave conclufions different from the truth at lower heights. He was hence led to conclude that the condensations of the atmofphere did not follow the fame laws at different heights; and formed an hypothefis to account for these variations, in which he fuppofes that the particles of air, at different heights from the earth's surface, are poffeffed of unequal degrees of elasticity. The reader may fee his obfervations on this head, in the memoirs of the R. Academy of Sciences at Paris, for the year 1753. If this hypothefis were true, it would be impoffible to apply the barometer to the menfuration of heights, with any degree of certainty. M. de Luc however has fhewn, by his more accurate experiments, that this pretended inequality of fpring in the particles of air does not fubfift; and that its condenfations and dilatations follow the fame laws uniformly at all heights, and in all climates; excepting only certain differences caused by heat, and other local circumstances.

Now it is one of the confequences of the above-mentioned principle, that if the heights of the mercury in the barometer be taken in a geometrical feries, the correfpondent heights of the air will be in an arithmetical progreffion. But the number of the terms of a decreasing geometrical progreffion being infinite; the correfponding defcents of the mercury in the barometer will form an infinite feries of terms decreafing in geometrical progreffion; and confequently, the refpective heights of the air, correfponding with them in an arithmetical progreffion, the height of the atmosphere, according to this principle, muft be infinite.

"See Seconde Suite des Memoires de l'Acad. Sc. Nouvelle Centurie. Tom. vi. p. 1770. Amfterdam edition.

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To fet fome limits therefore to this inquiry, the Author confines the problem to the affigning the vertical extent of the earth's atmosphere, at a certain fuppofed ftation, where the mercury in the barometer would ftand, for inftance, only at the height of one line. At fuch a height, the medium may nearly be confidered as a vacuum; or, the air will nearly be of the fame denfity with that to which we are capable of reducing it in the receiver of an air pump. This height however, it is to be observed, is fomewhat variable; as it is affected by the varying weight and heat of the air.

From data founded on accurate experiments, and by means of an easy calculation by logarithms, the Author finds that when the barometer ftands below at 27 inches, the thermometer at the fame time indicating o in the Author's fcale, (or 17° according to Reaumur's graduation) the height of the atmosphere, confidering the ftation above-mentioned as the limit of it, is 25,105,450 toifes, or 11 leagues and 3 toifes. It is easy to extend this calculation to any greater degree of rarefaction, or to a height where the mercury would fink to any given fraction only of a line. Thus for example, fuppofing the weight, &c. of the air below to be the fame as in the preceding inftance, the height of that region of the atmosphere, where the mercury in the barometer would ftand only at of a line, would be 35,105,450 toifes.

In treating of evaporation, the Author does not adopt the very plaufible theory of those, who attribute it to a diffolution of the fluid in air, as a menftruum; but offers various obfervations and argumen's to prove, that it is produced by a combination or union of the particles of fire, with those of the evaporating liquor. He has not however, in our opinion, obviated fome objections that may be made to this hypothefis. The journal of his experimental inquiries into the nature of ebullition, and particularly into the various phenomena attending the boiling of water, and the different degrees of heat, of which it is fufceptible under certain circumstances, contains many curious particulars. The perfeverance of the Author in the profecution of fome of thefe experiments, particularly in his attempts to deprive water of the air contained in it, is a phenomenon, we think, nearly as remarkable as any he records. In fome of these experiments, we find him fhaking a small portion of water, freed from the preffure of the air, and contained in a large thermometer or fmall matrafs, during the fpace of a month, in order to extricate and expel the air from it. During this whole procefs, the matrafs was scarce ever out of his hand, or fuffered to reft, except while our operator flept, or was employed in neceflary avocations that required the use of both hands. • I ate,

APP. Rev. Vol. 1.

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I ate, fays the Author, I read, I wrote, I converfed with my friends, I walked into the town, all the time thaking my water without ceafing; and at the end. of the time, it still continued to furnish air, after every fuccuffion.'-M. de Luc's perfeverance on this occafion appears to equal that even of Boerhaave, diftilling the fame identical portion of quickfilver 510 times. M. de Luc however was folely actuated, throughout the whole of this tedious routine, by the pure love of fcience: but Boerhaave's zeal was not quite fo difinterested, as it was excited and fuftained by the hopes of realizing the golden dreams of the alchemists.

Many curious circumstances attend his long process, which illuftrate fome of the properties of the inftrument described by Dr. Franklin in his letters, and of which we have pretty largely treated in our review of that performance*. We shall only relate the following refults of this laborious courfe of experiments; from which it appeared that a confiderable quantity of air is contained in water, which it obftinately retains; fo that it cannot be deprived of it either by boiling, or by the air pump, or by any other known means, than a long continued agitation in vacua; and that when it has been deprived of all the air that can be thus feparated from it, it acquires fuch a conftitution as enables it to fuftain, without boiling, a degree of heat much fuperior to that which can be given to it in its natural ftate. In that ftate, it is well known that common water boils at 212 degrees of Fahrenheit's thermometer; but after M. de Luc's long continued concuffions, and other operations upon it, it bore a heat of about 240 degrees before it boiled; and, in another experiment, it did not boil, till the oil, in which the little matrafs containing the water was immerfed, had acquired a heat of above 285 degrees, [112 Reaumur]. When it was thus deprived of its air, it would fuftain the heat of boiling water, without fhewing any figns of ebullition, though the preffure of the atmosphere was taken off from it. We fhould not omit to obferve that a confiderable part of the additional heat acquired by the water, under the foregoing circumftances, is to be attributed to the small degree of diffipation to which it was expofed, in confequence of the form of the veffel.

In treating of the cold produced on the evaporation of liquids t, the Author endeavours to account for this curious phenomenon by the following hypothefis. He fuppofes that fire does not enter Auids with the fame facility that it leaves them. If all bodies, he obferves, were fo conftituted that fire as easily

* See M. Review, Vol. xlii. March 1770, page 207, &c.
Ibid. page 206.

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penerated them, as it efcapes from them, the heat of every body would be equal to that of the neighbouring ones: but fluids, he fuppofes, though they readily admit the particles of fire, ftill more readily part with them. When fire enters into a liquid, he conceives that it pushes its particles from without, inwards; in which action it meets with refiftance, much greater than that which it fuftains on leaving the fluid, when it acts on the extreme particles, or thofe on the furface, in a direction from within, outwards, carrying fome of them off along with it. Every liquor therefore, he adds, that evaporates, ought to be cooler than the furrounding air; and this, in proportion to the rarity of the air, and the extent of its furface in contact with that of the fluid. Accordingly a very volatile liquor, placed under a receiver, in which the air has been greatly ra rified, and expanded over the furface of a piece of linen wetted with it, is in the most favourable fituation for the produc tion of artificial cold.

We shall take our leave of this ingenious performance by ob ferving that, after all the Author's laborious and accurate refearches with refpect to his principal object, or the menfuration of heights by the barometer; it may ftill be thought that there are few perfons who are qualified, or may be inclined to profecute this method, on account of the fmallnefs of the fcale, the many minutia to be observed, the different fources of uncertainty, and other difficulties attending the application of the barometer to this purpose. These difficulties have however in, a very great measure been removed by the perfeverance and fagacity of the Author; who firft by improving the barometer itfelf, and afterwards by detecting and afcertaining, by means of the inftrument thus improved, the effects of various caufes acting on the atmosphere, and which greatly affect the calculation, has enabled others ftill further to improve and facilitate this method, and to fupply the few defiderata ftill wanting to bring it to perfection. How nearly he has himfelf approached to it, is evident from numerous examples here given; in many of which the juftice of his calculations was afcertained by actual admeasurement by the line. On the whole it appears that the heights of different ftations, as calculated from that of the barometer, by his formula, have approached fo near to the actual heights, that the greatest differences, and those very few in number, do not exceed the one hundreth part of the whole.

In answer to the doubts that may yet be entertained on this head, and to ftimulate future inquirers and obfervers, we shall conclude with a quotation from the Author, immediately relating to this fubject.

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When I fee,' fays M. de Luc, the aftronomer in his obfervatory attempting to meafure the diftance of the stars, by comparing them with the minute, and almoft imperceptible divifions of his inftruments; viewing them, at the fame time, through a medium that variously refracts the rays of light:When I fee the geographer determining the pofition of places on the earth's furface, merely by that of his telescope fixed to the limb of his quadrant, and by a pendulum;-I do not hesitate to offer the small scales of the barometer and thermometer, as fit measures of acceffible heights, But at the fame time, I appeal to the geographer and aftronomer, whether they have perfected their refpective arts at once; and whether the exactnefs of the mathematician would have been of much service to them, had not his labours been seconded by those of the artist and the obferver.'

ART. X.

Explication de quelques Medailles, &c.-An Explanation of certain Greek and Phenician Medals. By M. L. Dutens. Quarto. London. Thane. 1773.

ART. XI.

Explication, &c.-An Explanation of fome Phenician Medals, in the Cabinet of M. Duane. By M. L. Dutens. Quarto. London. Thane. 1774.

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HOUGH we have of late been repeatedly tempted, and have as often taken occafion, to treat certain elaborate and folemn difcuffions of matters relating to antiquity, with an air of levity, feemingly ill fuited to the gravity of the fubject; yet we are very ready to declare that we refpect every flip and corner of the extenfive fields of fcience and literature too highly, to involve all thofe, whofe lot or choice it may be to cultivate even the most barren fpots of either, in one indifcriminate cenfure. When the investigation of antient coins, or other monuments of antiquity, tends to the discovery of new facts that have the leaft claim to fignificance ;-when it leads to the elucidation of an obfcure or contraverted point of hiftory; when it points out the progrefs, ftate, and declenfion of the arts among a people ;-in fhort, whenever it gratifies a laudable curiofity, or contributes in any degree to the ad-vancement of any branch of useful, or even ornamental knowledge; it is in no danger of incurring our animadverfion or ridicule provided neverthelefs, that fuch ridicule is not extorted from us, by circumstances of a rifible quality, the operation of which it is impoffible for all the phlegm even of a reviewer to refift.

M. Dutens, our readers may recollect, is the Author of an ingenious work, in which he endeavoured to fupport the pri

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