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wheels over the balance. If a clock be stopped, the pendufum must be raised from reft through a certain arch, before the swing wheel can escape, and the clock go again I. In like manner if a watch be stopped, the balance must be drawn round from its place of rest to a certain degree, before the watch can go again. If the pressure of the main spring (through the wheels) upon the balance, can sufficiently force the balance from its place of rest (against the effort of the balance spring to bring it back), the main spring will then be always able to fet the watch a-going if stopped. But to do this, it is evidene the power of the main spring on the balance, muft equal the power of the balance spring, at the place of escapement. It follows therefore, if a watch be so constructed as to set itself 2-going when stopped, it cannot possibly be a good time-keeper, because the wheels muft then have so great a dominion over the balance 5. The principles Mr. H. proceeded upon therefore, instead of serving to improve pocket-watches, as fome expected before the discovery was made; ferve only to show that such watches are incapable of great improvement. Improvement is incompatible with a very necessary property in a pocket-watch, that of setting itself a going when stopped. Nor will Mr. H. allow to such a bauble as a pocket-watch, the respectable appellation of a TIME-KEEPER 1.

(To be concluded in our Review for Auguft.] 1 The arch through which the pendulum or balance must be raised from its lowest point, or from the place of rest, that the tooth of the wheel may escape from under the palet, is called the arch of escapement. by che French l' arc de levée.

It is said Mr. H. boasted in the H. of C. that if his time keeper was stopped, it would not see itself a-going again,-to the amaze. ment of the members, who could by no means comprehend the benefit of such a property:

1 “ In common watches the wheels have about one-third the dominion over the balance that the balance spring has, but in this my timekeeper the wheels have only about one-eightieth part of the power over the balance that the balance spring has; and it must be allowed the less the wheels have to do with the balance the better."-Prin.. ciples of Mr. Harrison's Time-Keeper, page 20.

ART. IX. Recherches sur les Modifications, &c. Inquiries into the different Modifications of the Atmosphere, &c. By J. A. De Luc. Concluded.

[From the Appendix to our latt Volume, Page 579.] N consequence of the large extracts which we have already

made from this elaborate performance, we thall content ourselves, in this our concluding account of it, with giving the substance of a few miscellaneous observations selected from

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it. And as our philosophical readers may wish to know the real height or extent of the earth's atmosphere, as deduced from a long course of experiments, made with the most perfect inftruments, and the results of which have been calculated with a degree of precision never perhaps before attained in this inquiry; we shall, in the first place, give them the substance of his final conclusions on this subject; premising only a fhoit account of the material principles, or elements, on which they are founded.

M. De Luc parts from this well known postulatum ; that the density of the air is every where proportional to the weight with which it is pressed. M. Bouger however was induced to contelt the truth of this fundamental propofition, on finding that his formula, deduced from numerous observations made in the superior parts of the atmosphere, and which agreed with actual observations made in the upper parts of the cordeliers, gave conclusions different from the truth at lower heights. He was hence led to conclude that the condensations of the atmosphere did not follow the same laws at different heights; and formed an hypothesis to account for these variations, in which he supposes that the particles of air, at different heights from the earth's surface, are pofleffed of unequal degrees of elasticity. The reader may see bis observations on this head, in the nemoirs of the R. Academy of Sciences at Paris, for the year 1753. If this hypothesis were true, it would be impoffible to apply the barometer to the mensuration of heights, with any degree of certainty. M. de Luc however has îhewn, by his more accurate experiments, that this pretended inequality of spring in the particles of air does not subfilt; and that its condensations and dilatations follow the same 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 consequences of the above-mentioned principle, that if the heights of the mercury in the barometer be taken in a geometrical series, the correspondent heights of the air will be in an arithmetical progreffion. But the number of the terms of a decreasing geometrical progression being infinite; the corresponding descents of the mercury in the barometer will form an infinite series of terms decreasing in geometrical progression; and consequently, the respective heights of the air, corresponding with them in an arithmetical progreffion, the height of the atmosphere, according to this pria. ciple, must be infinite.

. See Seconde Suite des Memoires de l'Acad. &c. Nouvelle Ceaturit. Tom. vi. p. 1770. Amiterdam edition.

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To set some limits therefore to this inquiry, the Author confines the problem to the affigning the vertical extent of the earth's atmosphere, at a certain fupposed station, where the mercury in the barometer would stand, for instance, only at the height of one line. At such a height, the medium may nearly be considered as a vacuum ; or, the air will nearly be of the same density 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 somewhat 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 stands below at 27 inches, the thermometer at the same time indicating o in the Author's scale, (or 19° according to Reaumur's graduation) the height of the atmosphere, considering the station above-mentioned as the li. mit of it, is 25,105;450 toises, or 11 leagues and 3 toises. It is easy to extend this calculation to any greater degree of rarefaction, or to a height where the mercury would link to any given' fraction only of a line. Thus for example, fuppofing the weight, &c. of the air below to be the same as in the preceding instance, the height of that region of the atmosphere, where the mercury in the barometer would stand only at t'u of a line, would be 35,105,450 toises. • In treating of evaporation, the Author does not adopt the very plausible theory of those, who attribute it to a diffolution of the Auid in air, as a menstruum ; but offers various obfervations and arguments 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 some objections that may be made to this: hypothesis. 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 perseverance of the Author in the prosecution of some of these 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 some of these experiments, we find him taking a small portion of water, freed from the pressure of the air, and contained in a large thermometer or fmall matrass, during the space of a month, in order to extricate and expel the air from it. During this whole process, the matrass was scarce ever out of bis band, or suffered to rest, except while our operator slept, or was employed in neceflary avocations that required the use of both hands.

APP. Rev. Vol. 1.

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. I ate,

510 times.

. I ate, says the Author, I read, I wrote, I conversed with my friends, I walked into the town, all the time thaking my water without ceasing; and at the end of the time, it ftill continued to furnish air, after every succussion.'-M. de Luc's perseverance on this occasion appears to equal that even of Boerhaave, distilling the fame idențical portion of quicklilver

M. de Luc however was solely actuated, throughout the whole of this tedious routine, by the pure love of fcience : but Boerhaave's zeal was not quite so disinterested, as it was excited and fultained by the hopes of realizing the golden dreams of the alchemists.

Many curious circumstances attend his long process, which illustrate fome of the properties of the instrument 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 results of this laborious course of experiments; from which it appeared that a confiderable quantity of air is contained in water, which it obstinately 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 conti. nued agitation in vacao, and that when it has been deprived of all the air that can be thus separated from it, it acquires fuch a constitution as enables it to sustain, without boiling, a degree of heat much superior to that which can be given to it in its natural state. In that state, it is well known that common water boils at 212 degrees of Fahrenheit's thermometer ; but after M. de Luc's long continued concussions, 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 matrass containing the water was immersed, had acquired a heat of above 285 degrees, (112 Reaumur). When it was thus deprived of its air, it would fustain the heat of boiling water, without shewing any figns of ebullition, though the pressure of the atmosphere was taken off from it. We thould not omit to obferve that a considerable part of the additional heat acquired by the water, under the foregoing circumftances, is to be attributed to the small degree of dislipation to which it was exposed, in consequence of the form of the vessel.

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

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

penetrated penerated them, as it escapes from them, the heat of every body would be equal to that of the neighbouring ones: but fluids; he supposes, though they readily admit the particles of fire, Nill 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 resistance, much greater than that which it sustains on leaving the Auid, when it acts on the extreme particles, or those on the surface, in a direction from within, outwards, carrying some of them off along with it. Every liquor therefore, he adds, that evaporates, ought to be cooler than the surrounding air; and this, in proportion to the rarity of the air, and the extent of its surface in contact with that of the Auid. Accordingly a very volatile liquory placed under a receiver, in which the air has been greatly ra rified, and expanded over the surface of a piece of linen wetted with it, is in the most favourable fituarion for the produce tion of artificial cold.

We shall cake our leave of this ingenious performance by observing that, after all the Author's laborious and accurate researches with respect to his principal object, or the mensuration of heights by the barometer ; it may still be thought that there are few persons who are qualified, or may be inclined to prosecute this methud, on account of the smallness of the scale, the many minutiæ to be observed, the different sources 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 perseverance and sagacity of the Author; who first by improving the barometer itself, and afterwards by detecting and ascertaining, by means of the instrument thus improved, the effects of various causes acting on the atmosphere, and which greatly affect the calculation, has enabled others still further to improve and facilitate this method, and to supply the few desiderata ftill wanting to bring it to perfection. How nearly he has himself approached to it, is evident from numerous examples here given; in many of which the justice of his calculations was ascertained by adual admeasurement by the line. On the whole it appears that the heights of different stations, 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 observers, we shall conclude with a quotation from the Author, immediately relating to this subject.

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