tain 109 pages, is entirely employed in the investigation of the origin of Italian poetry, and more efpecially in treating of the language, learning, and genius, of Dante; alfo of his imitations, and his imitators. M. ART. X. Defcription of an improved Air-pump; and an Account of fome Experiments made with it: By JoHN CUTHBERTSON, Mathematical Inftrument Maker in Amfterdam. 8vo. Amfterdam. 1787. HOUGH the air-pump was invented above a century ago, it has not been brought to that degree of perfection which might have been expected from its long and generally acknowledged utility in phyfical inquiries. It was indeed greatly improved by the ingenious Mr. Smeaton; but even his conftruction left feveral imperfections unremedied. One of these was occafioned by the preffure of the pifton upon the valve that opens to let the air into the barrel from the receiver. This defect Mr. Cuthbertson removed fome years ago, by placing this valve on one fide of the barrel; and of this improvement he gave a defcription, in the Dutch tranflation of Dr. PRIESTLEY'S Obfervations and Experiments on different Kinds of Air, published in 1781. With pumps of this conftruction, dry air might be rarified. about fix hundred times: this feemed to be the highest degree of perfection of which the pneumatic engine was capable; for the reason of its not exhaufting further was evidently the weakened elafticity of the air remaining in the receiver, which decreasing in proportion as the quantity is diminished, becomes at length infufficient to raise the valves that open a communication between the receiver and the barrels, fo that no more air can pafs from the former to the latter. This defect was, however, in fome measure removed by Mr. Haas, who, by means of a pin fliding through a collar of leathers, enabled the operator to push up the undermoft valve, when the elafticity of the air was too much diminished to raise it. But, for want of a fimilar contrivance to open the valve in the pifton, which was equally neceffary, the improvement was incomplete. Befide, the additional mechanifm rendered the machine more liable to become leaky, and thus increased an inconvenience which is common to all air-pumps that are made with valves. Thefe imperfections feem to be entirely removed by the ingenious contrivance of Mr. CUTHBERTSON, whofe air-pump has neither cocks nor valves, and is fo conftructed, that what fup plies their place has the advantages of both without the inconveniences of either. A particular defcription of the mechanism of the inftrument would not be eafily comprehended without the plates. Mr. CUTHBERTSON, not finding the long barometer gage, as it is commonly ufed, fufficiently exact, has improved it, by immerging, in the fame ciftern, a barometer tube, filled with mercury well purified from air: these tubes are bound together, fo as always to ftand parallel to each other, and furnished with a fliding fcale, by which the difference between the height of the mercury in the gage and barometer tubes may be difcerned with the greatest exactness, even to the hundredth part of an inch. Another gage here described is of his own invention, and is in the form of a double fyphon, in which the degree of exhauftion is eftimated alfo by the difference between the height of the mercury in the two legs. Thefe gages were found always to indicate the fame degree of rarefaction. It is well known, that if the receiver be placed upon leather, either oiled or foaked in water, this, when the preffure of the air is diminished, will yield an expanfible vapour, which will drive out a great part of the remaining permanent air, and fill up its place in the receiver; but our Author has found that none, or at leaft very little of this vapour, is yielded by leather dreffed with allum, if it be foaked in hogs-lard: this therefore he recommends in common experiments; but when the utmost degree of exhauftion is required, his advice is, to dry the receiver well, and fet it upon the plate without any leather, only fmearing its outfide edges with hogs-lard, or with a mixture of three parts of hogs-lard and one of oil. The ufe of the leather has long been laid afide by our English mathematical inftrumentmakers, a circumftance which probably has not come to Mr. Cuthbertfon's knowledge. As in this machine there are no valves to be forced open, nor any thing to prevent the air in the receiver from expanding itself to its utmoft degree, it is eafy to conceive that its exhaufting power must be much greater than that of air-pumps as they are commonly conftructed. With this, however, as with all other inftruments of the kind, the degree of exhauftion that can be produced is different at different times, and depends much on the ftate of the atmosphere, being always greatest when this is moft free from moifture. From the experiments here related, it appears, that in fine weather the barometer and fyphon gage may generally be made to indicate a rarefaction of twelve hundred times; but, when the atmosphere was very dry, the exhauftion. has been fo complete, that the gages have fhewn the air in the receiver to be rarefied above twenty-four hundred times. Thefe These air-pumps are fo conftructed, that they may be made to condenfe, either at the fame time that they exhauft, or feparately; in the former cafe, the air, exhaufted out of one receiver, may be forced into another; and thofe with two barrels are fo contrived, that either of thefe may be used independently of the other. Befile the experiments which tend immediately to fhew the great degree of rarefaction which may be effected with this machine, Mr. CUTHBERTSON has related fome, in which the electric fluid was made to pafs through a glass tube two feet and an half long, while under exhauftion. When the air in this was rarefied fourteen hundred times, it conducted fo well, that the fparks from the prime conductor connected with it, which, before exhauftion, had been two inches, were only one fortieth part of an inch in length; and in the middle of the tube there was a space, fix inches long, entirely void of light, which at its two extremities appeared of a faint white colour. Our Author has alfo added fome experiments on the elaftic vapour which is emitted in vacuo by leather and other moist fubftances. One of thefe, in which this expanfible fluid is vifibly diftinguished from air, is fo ingeniously contrived, that we shall conclude this article with an account of the manner of performing it. For this purpose, the air muft be made to pafs from the barrel into the atmosphere through an inverted glafs fyphon, about half full of water; let a piece of leather dreffed in allum, about an inch fquare, be tied to a piece of lead; and, putting it into a glafs, pour upon it tranfparent lamp-oil, about half an inch higher than will cover it. This being fet upon the plate under a receiver, and gradually exhaufted, the leather will emit the air contained in it, which will be feen to rife in bubbles through the oil into the receiver, whence it paffes through the barrels of the pump into the fyphon, and will afcend in bubbles through the water, into the air. But, when the preffure of the air in the receiver is so far diminished, that the gages become stationary, the expanfible fluid contained in the leather will affume the form of air, and likewife afcend, through the oil, into the receiver; but it will be found that, though the bubbles rife very quick, and in great abundance, through the oil, none will be seen, as before, to afcend in the water; because the elaftic vapour, as foon as it enters the fyphon, is condenfed by the preffure of the atmosphere, and reduced to its former. ftate, in which it cannot produce any fenfible effect. ART. XI. Hiftoire d'Elifabeth, Reine d'Angleterre, &c. i. e. The History of Elizabeth Queen of England, compiled from original English Writings, Acts, Deeds, Letters, and other Manufcripts, never before published. By Mademoifelle de Keralio. Vols. I, II, and III. 8vo. Paris. 1787. MAN ANY circumstances concur to render it highly probable, that fome important political revolution, in France, is at no great diftance. The fyftem of government which there prevails, is by no means adapted to the enlightened fpirit of the people. The clofe intercourfe which has long fubfifted between literary perfons in that country and in this, made it impoffible to prevent our neighbours from imbibing, from the writings and private correfpondence of our countrymen, the principles of freedom, and an interefting idea of the natural rights of mankind: nor can thofe ideas, when they have once taken poffeffion of the human mind, be ever eradicated by any exertion of authority or defpotifm. Fear, indeed, may in fome measure prevent them from being freely expreffed in public; but that very fear will tend to caufe them to be cherished, with an increafing degree of favour, in private nor is it, perhaps, poffible, by any ftretch of power, in a kingdom of fuch extent, and fo circumftanced as France, to prevent those who have a strong conviction of the injury they fuftain by being deprived of fome of the most natural privileges of men, from communicating their ideas, in one way or other, to their bofom friends and intimate acquaintance. Like a fmothered flame, therefore, this kind of contagion may fpread far and wide before it is difcovered, and at the moment it openly breaks forth, it may have acquired fo much ftrength as to baffle all attempts to reprefs it. The experience of more than half a century paft, ought to have convinced the French government, that fuch a spirit of freedom was fecretly cherished among perfons of liberal education in France, and that, though this was in fome measure repreffed among them by a refpect for the King, and a defire to preferve good order in the ftate, yet it might have been obvious, that if occafion should ever be given to bring fpeculations of this fort within the reach of the vulgar, it would be a matter of the utmoft difficulty to retain them within the bounds of moderation. and decorum, or to prevent them from afferting their rights by every means within their reach. Thefe confiderations ought certainly to have induced the rulers of France to have acted, on a late occafion, with an extraordinary degree of caution, if they meant to preferve undiminifhed the regal authority in that country. But men will ever be influenced by paffions and prejudices, which will frequently ftifle, for a time, the fuggeftions of found APP. Rev, Vol. LXXVII, Qq policy. policy. When the French adminiftration faw Britain engaged in a ftruggle with her colonies, concerning the extent of her jurifdiction over them, the spirit of inimical rivalship prevailed over every other confideration, and the opportunity appeared fo favourable for humbling that dreaded rival, as not to admit of a doubt about the propriety of their openly appearing in fupport of thofe colonies. Inftead, therefore, of fecretly affording the Americans a fmall fupport, that might have ferved to protrac the difpute, and thus probably at laft effect the purpose they aimed at, they fent numerous bodies of French troops, to act, for many years together, in a country where queftions concerning the prerogatives of fovereignty, and the rights of the subject, were agitated with the moft unbounded freedom, among all claffes of men; and in which difputes the arguments in favour of the people were fure to prevail. In confequence of that interference, Britain, happily for herfelf, was forced to relinquifh a fovereignty, which ferved only to reprefs her own internal induftry, and retard her profperity; while France, in return for the boon fhe had thus undefignedly conferred on her rival, imported into her own dominions, at the clofe of the war, an hoft of political reformers, whose zeal cannot be fuppofed to be moderated by prudence, and who, by being difperfed through all the provinces of the kingdom, muft foon obtain fuch a numerous body of profelytes as will quickly command refpect. The fruits of this plan of conduct begin already to appear, and will in the end, we doubt not, be productive of much good to the people of that extenfive and refpectable kingdom, though we fear the ftruggle for power on both fides may for a time tend to disturb the public tranquillity. The profpect of these troubles may perhaps afford pleafure to political fpeculators; for our own part, as members of the republic of letters, whose native country is THE GLOBE, we can take pleasure in nothing that tends to difturb the tranquillity of mankind; and we shall therefore rejoice to fee a speedy termination of these difputes, by a liberal extenfion of thofe privileges in France, which have tended fo much to promote fcience, and to the improvement of useful arts in Britain. The work before us affords, we would fain hope, a happy prefage of the near approach of that desirable period, which we to fincerely with to fee. Like many other productions of French authors, which do honour to the republic of letters, the Hiftory of Elizabeth, by Mademoiselle de Keralio*, abounds with just *This lady is, we fuppofe, the daughter of M. de Keralio, member of the Academy of Belles Lettres, &c. and author of the Hiftory of the Wars between the Ruffians and Imperialifts against the Turks, in 1736, 7, 8, and 9. See Review, vol. lvi. p. 52. reflections, |