A Tinman at Seville, ignorant of the general principles of hydroftatics, and equally ignorant of the bounds which nature has fet to the afcent of water in vacuo, as of the cause of its elevation, confidently undertook to convey it up to a terrace 60 feet high, by means of a fimple fucking-pump. Having completed his apparatus, he falls to work upon the top of the terrace; but the water refuses to obey his repeated fuctions. Irritated at this difappointment of his hopes, he runs down in hafte, and in a fit of rage and defperation throws a hammer, which he held in his hand, at the pipe, with fuch good-will, and in fuch a direction, as to make a hole in it, at the height of about 10 feet above the refervoir; and, in confequence of this lucky hit, the water inftantly rushes up to the place of its deftination. Thus chance produced what genius had never yet effected, and a passionate blockhead, by a dash of a hammer, drove water up through the pipe of a fimple fucking-pump to the height of 60 feet, which before, and ever fince the days of Torricelli, would never proceed much farther than 30.-In a fomewhat fimilar manner, a painter, we have fomewhere read, after many fruitless attempts to paint the foam about the mouth of Bucephalus, dafhed his pencil in a rage against the picture, where initantly an excellent foam appeared, when he leaft expected it.

But it is incumbent on us to explain, if the philofophical Reader has not already anticipated us, the caufe of this effect. It ap pears evidently, from the Abbé Nollet's experiments, that the preffure of the atmosphere does not, in this cafe, raife a folid or continuous column of water 50 or 60 feet in height, or, in other words, a weight greater than its own; but that it only elevates a compound column of water and air intermixed, which is confiderably lighter. In fact, the water having been first raised to its usual height, by the rarefaction of the air within the tube, the external air rufhes in through the fmall artificial aperture; and while it depreffes the water below the orifice, which confequently falls into the bafon, having now only the weight of a column of water 20 feet high above it, that is, ds only of the weight it is able to fuftain, it elevates it, but at the fame time however in its paffage upwards becomes intermixed with it; and the whole compound mass of air and water is, by the continued preffure of the atmosphere, carried up to the height of 50 feet above the aperture.

Chance, as we have already mentioned, gave rife to this ob fervation; though the effect might undoubtedly, we think, have been conje&ured a priori. It is well known, that on lifting up the tube of a barometer out of the bafon, fo as to permit a part of the mercury to fall out, and of air to enter, the remaining column, now become lighter than the atmosphere, is fuddenly elevated and dashed against the top of the tube: fo that the most remarkable

remarkable circumftance in the Seville experiment is, that the water, inftead of being elevated to fo confiderable a height, does not rather defcend through the air, a fluid fo much lighter than itself; while the latter might naturally be expected to rush through it to the top of the tube. This event however does not take place, when the tube is of a proper bore, and the aperture is made at a certain determinate height above the furface of the refervoir. It may be proper to obferve, that in the Seville pump, as well as in M. Le Cat's imitation of it, the ftream at the top is intermittent; and that it is neceffary, after the first discharge of water, to stop the hole for a short time, in order to procure a fecond; or, in other words, that the hole must be alternately stopped and opened by an assistant, or otherwife, during the working of the pump.

Soon after the Abbé had cleared up this hydroftatical paradox, the truth of his explication of which he afterwards confirmed by experiments made with glafs tubes, in the presence of the Academy, he was informed of ano her fingular machine of the fame kind, which excited more furprize than the former. He was told that the Sieur Bellangé at Paris had actually conftructed a simplefucking-pump, which not only raised water to the height of 55 feet, but delivered it in an uninterrupted ftream, as long as it was worked, without requiring any attendance at the little apperture. He made a vifit to that artist, and found that the performance of his pump was fuch as it had been reprefented to him. The bore of the pipe of this machine was 10 lines in diameter, and that of the little aperture, which is by no means a matter of indifference, half a line. The latter was pierced at the distance of a foot from the furface of the water, and had a flender valve, which was fixed on the infide; although the machine would work, if the aperture remained always open. On applying his ear to this opening, the air was heard rufhing in through it with a hiffing noife; and from this, and other circumftances, he wast convinced that the pump did not raife a continuous body of water 55 feet in height, but an interrupted column, confifting of alternate maffes of water and air. On this account, the quantity of water delivered by it falls very much fhort of what might otherwife be expected, and renders it, at leaft in its present state, rather a matter of curiofity than of much ufe. Nevertheless, there may be fome circumstances in which this conftruction may be employed to advantage; particularly where the fource is inacceffible, or fo fituated as to render the fixing of any of the common pumps difficult or impracticable.

MEMOIR II. On the Motion of Finids running through given Apertures in the Bottom of a Veffel. By M. Le Chevalier De Borda. In queftions of pure geometry, the feience of certainty, no difference can arife in the folutions of geometricians: but when

they

they are complicated with phyfical confiderations, the minuteft circumftance added, overlooked, or neglected, is fufficient to produce error in the refults. The problem which is the fubject of this Memoir has been profoundly inveftigated by several great geometricians; particularly by Meffrs. Daniel Bernoulli, and D'Alembert. M. De Borda, however, thinks their folutions in fome respects erroneous, and here endeavours to fubftitute others more just and accurate, founded on a different hypothefis, and on actual experiments.

We shall only give the titles of the two remaining Memoirs of this class. The first treats of the different methods of laying the foundations of bridges and other works under water: the fecond contains an account of the fcheme of bringing the waters of the river D'Yvette to Paris, together with proofs of their falubrity.

ASTRONOMY. MEMOIR I. and II. On the Theory of the Planet Mercury. By M. De la Lande.

Thefe Memoirs are curious, not only as aftronomical articles, but likewife on account of the critical difcuffions, relative to a curious and interefting part of ancient literature, contained in them. We shall accordingly give a fhort and general abstract of their contents.

The theory of the planet Mercury, it is well known, is far from having been carried to that degree of perfection, which has been attained to with regard to that of the other planets. This imperfection has evidently arifen from the great rarity and infufficiency of obfervations; and thefe have been caufed by the extreme fmallness of that planet, and his vicinity to the fun even at his greatest elongations, which render him frequently indifcernible even by modern aftronomers, provided with the beft inftruments; efpecially in his paffage over the meridian, where M. De la Lande has frequently not been able to observe him, with the affiftance of a large reflector moving in the plane of that circle. We are not to wonder, therefore, that the antient obfervations of this planet amount only to fixteen, which are all contained in the only monument of the antient aftronomy which is come down to us, the Almageft of Ptolemy; and thefe we owe to the favourable fituation as well as industry of the obfervers. Copernicus, fituated in a more northern climate, and confequently in a more oblique fphere, and incommoded by the fogs arifing from the Viftula, lamented that he could never once obtain an obfervation of this planet. Tycho Brahe, Hevelius Riccioli, and other more modern aftronomers, were, however, more fortunate. After difcuffing the refpective merits and defects of their obfervations, M. De la Lande produces a few lately made by himself, under favourable circumitances and fiAPP. Rev. vol. xliv.

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tuations

tuations of the planet, and from thence corrects the errors of the tables, and deduces the place of his aphelion for the present time.

In the fecond Memoir, the Author undertakes to determine the motion of the aphelion, and the mean motion of Mercury, together with the mean time of his revolution, and his distance from the fun. For thefe purposes he enters into a critical examination of the fixteen obfervations abovementioned, contained in the Almageft; feveral of which he judges to be of the greateft importance, and to be as capable of affording a determination of the motion of the aphelion, as well as of fome of the other elements relative to the theory of this planet, as exact at leaft as all the obfervations made in the last century. The numerous difficulties which the Author meets with in this undertaking, give him frequent opportunities of difplaying not only his well-known aftronomical knowledge, but likewife his erudition; and of exerciting his critical difcernment, in correcting the unavoidable errors which have crept into that precious monument of antiquity, during a course of more than fixteen centuries, through the faults of copy ifts and translators, and afterwards thofe of printers. The manufcripts of the Almageft, in the original Greek, were for many ages loft to the world but the Arabs had tranflated this work into their language, and from thefe Arabic verfions were made the Latin tranflations which we now poffefs. Long afterwards, however, a copy of the original Greek was discovered, and publifhed towards the beginning of the 16th century, from the fifth edition of which the Author tranfcribes the fixteen obfervations which are found in it, made by Ptolemy and his predeceffors, relative to this subject; correcting the text where it is faulty, and elucidating it where it is obfcure.

Of the difficulties attending this undertaking, fome arife from the manner of computing time ufed by this antient writer; and from obfcurities in expreffion both on that and other fubjects. The Egyptian year, and the Era of Nabonaflar, ufed by Ptolemy, M. De la Lande firft reduces to the Julian year, and the vulgar Æra. In oppofition to fome of the learned, who have erred on this head, he fhews that the first day of the first year of Nabonaflar falls precifely on the 26th of February, 747 years before Jefus Chrift; a date inconteftibly established on aftronomical principles, and on the authority of Ptolemy himself, who gives the places of all the planets for that day: on which occafion M. De la Lande obferves, that no other day or year can poffibly agree with all their different longitudes (particularly that of the moon) except thofe above indicated. By calculating the fun's places, and comparing them with the actual obfervations as given by Ptolemy, he determines a point which was doubtful, but which

was

was neceffary to be afcertained, with regard to his manner of reckoning the day, and finds that he began the day, as modern aftronomers do, at noon; and not, as the antient Egyptians did, at fun-rife. Finally, among other ambiguities relative to the text of this Author, he fettles that important one arifing from his frequent manner of expreffing the diftances of Mercury from the fixed ftars, which are not given in degrees and minutes, but in moens, half-moons, thirds of moons, &c.; that is, in diameters of that planet, and in parts of that diameter. M. De la Lande, by a delicate calculation, difcovers the precife value which Ptolemy affigned to that meafure; and finds his lunar diameter to have been equal to 32'. 45". He expreffes his furprize how, without the ufe of telescopes and micrometers, he could approach fo near to the true measure of the moon's diameter. He fuppofes it however not to have been obtained by actual measure, but deduced from calculations of eclipfes. The Author having, in thefe two Memoirs, determined all the other elements of Mercury, except the equation of the centre, propofes to render his theory complete, by a determination of that element, which is to be the fubject of a future Memoir. MEMOIR III. On the Motion of the Nodes, and the Variation in the Inclination of the Orbits of the Satellites of Jupiter. By Mr. Bailly.

In our xxxviith volume (Sep. 1767. p. 167.), and in the Appendix to the xlii, p. 503, &feq. we have given a fhort account of Mr. Bailly's attempts to perfect the theory of the fatellites, by calculations and reafonings founded on the theory of gravitation; with which the variations obferved in their inclinations had by fome been fuppofed to be incompatible. In this Memoir he continues the investigation, and finds almost every where a perfect coincidence between the refults of the calculations grounded on the Newtonian fyftem, and the best actual obfervations.

MEMOIR IV. On the Horizontal Refraction. By M. Le Mon

nier.

In this Memoir M. Le Monnier propofes a new method of afcertaining the quantity of the horizontal refraction, and its fmalleft variations, with the greateft precifion, by means of fome of the circumpolar ftars of the firft magnitude, which remain a fhort time under the horizon, obferved at the time of their rifing and fetting. To be more particular; his method confifts in measuring the arch of the horizon intercepted between the two points at which the ftar rifes and fets; and he propofes the bright star in Lyra as the moft proper for thefe obfervations in the meridian of Paris. To give an idea of the great precision of this method, we fhall add that, according to his calculations, fuppofing

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