the intire crown of aberration ought to appear, and the limb of Venus ought to appear within that of the fun, by a quantity equal to the whole breadth of this luminous ring.

As to the white luminous ring, obferved by fome to furround the planet while fhe was on the fun's difk, M. de la Lande is of opinion that if it was not an optical illufion, or did not proceed from fome defect in the telescopes, the cause of it is to be fought for, in the proper atmosphere of Venus.-Without meaning to conteft the existence of an atmosphere round Venus, the phenomenon, we ftill think, may be fatisfactorily accounted for, independent of that fuppofition, by the caufe which we fuggefted in our 42d volume; (May 1770, page 399.) where we propofed an experiment that illuftrates and confirms our folution. To explain this matter further, we fhall add that after having long and attentively viewed a black circle placed on a light ground, or the dark body of Venus on the fun's difk, thofe parts of the Retina on which the dark circular image had been thus long received, and which had thereby been guarded against the light, are rendered more peculiarly fenfible to its impreffions than any other part of that membrane. Accordingly, on the leaft defigned motion, or even unperceived and unavoidable wandering, of the eye, over the disk of the planet; those more irritable parts of the Retina that are within, and near, the circumference of the dark image, will be exposed to the forcible impreffions of the light, reflected from the white ground immediately circumfcribing the dark body, and mult confequently convey to the mind the idea of a luminous border furrounding the dark circle. To the foregoing folution it may be objected that the luminous ring will conflantly appear to every one who tries the experiment, with due perfeverance, on paper; whereas the phenomenon of the luminous border round Venus was not obferved by all those who viewed the tranfit. But this difference may justly be attributed to various circumstances, the principal of which are that fome obfervers may have viewed the fun's difk through too dark a glass ;-or may not have kept the eye invariably fixed on the planet a fufficient time to produce the effect;-or laftly, not having expected, they may not have attended to, the phenomenon.

MEMOIR III. On the Quantity of the Sun's Parallax, as deduced from the Tranfit of Venus in 1769. By M. De la Lande. MEMOIRS IV. and V. On the Obfervation of the Tranfit made by the late Abbé Chappe, in California. By M. Caffini de Thury, and M. De la Lande.

MEMOIR VI. A Critical Examen of the Obfervations of the Tranfit of Venus in 1769, and of the Confequences which may be deduced from them. By M. Pingré.

Out of the great number of observations of the late Tranfit of Venus, M. de la Lande, in the third Memoir, felects two, APP. Rev. Vol. 1.

in

in each of which both the ingrefs and egrefs of Venus, or the two contacts, had been obferved. According to the method: employed by him, which does not require any precife knowledge of the longitude of the two places, but which depends on the chord defcribed by Venus's path over the fun's disk, he concludes the fun's horizontal parallax to be about eight feconds, inftead of ten, which had been formerly fuppofed.

In the fourth Memoir, M. Caffini gives a circumstantial account of the late Abbé Chappe's obfervations relative to the fun's parallax, made in California; where, as we have formerly at large related, the latter fell a martyr to his zeal for the determining of this important element. In the fifth Memoir these valuable and accurate observations are examined by M. de la Lande, who on comparing them with thofe of other Obfervers, by the method abovementioned, concludes that all of them concur in giving the parallax within very narrow limits; fo that we may, without any fenfible error, fix it at 8 feconds and. Employing this parallax, M. de la Lande has calculated the refpective diftances of all the planets, their diameters, maffes, denfities, and the velocities of falling bodies at their furfaces; and has given, if the expreffion may be allowed, a plan and elevation of the intire structure of the folar fystem, in a table which terminates this Memoir.

In the fixth Memoir, M. Pingré, after an examination of all the capital obfervations, including thofe made at Otaheite, infers the fun's parallax on the 3d of June to have been 8′′ 75, and concludes it to be, at the fun's mean distance, equal to 8" 88.

The remaining articles of this Clafs are, a Memoir of M. Caffini the younger, on the theory of the Comet which appeared in August 1769: a continuation of M. Du Séjour's profound investigation of the doctrine of Eclipfes; being his eighth Memoir on the fubject: fome remarks on the longitude of Cape François; and a fupplement to fome ancient obfervations of the longitude of the moon, compared with the tables.

DIOPTRICS.

MEMOIR. A Determination of the Refractive and Difperfive Powers of Crown Glafs and Flint Glafs, &c By M. Jeaurat. We have in many of the late volumes of our journal fucceffively given an account of the attempts of the different Members of the Royal Academies of Paris and Berlin, to afcertain the principles, and improve the conftruction of that excellent invention, the Achromatic Telescope. In this Memoir, M. Jeaurat gives the refult of his experiments and calculations on this fubject, in eight different tables, for the advantage of artifts who may not be qualified to profecute thofe delicate ex

See Appendix to our 48th vol. page 560.

periments

periments and calculations which this curious and difficult fubject requires.

M. Jeaurat proceeds on this principle; that as it is impoffi ble totally to annihilate both the aberration proceeding from the spherical figure, and that caufed by the different refrangibility of the rays of light; it is of the greatest importance to destroy that particular aberration which is the greatest and the most prejudicial, viz. the aberration of refrangibility. It happens fortunately that by the fame combination of lenfes, formed of different kinds of glafs, and of different and contrary figures, alternately difpofed, by which the aberration of refrangibility is deftroyed, that of fphericity is at the fame time diminished. For thefe and fome other reasons he turns his whole attention to annihilate the aberration of refrangibility.

M. D'Alembert has indeed affirmed that if this last aberration is to be intirely annihilated, the curvature of the combined lenfes must be fo confiderable, that it will be impoffible to give the object glafs of a telescope conftructed for this purpose a fufficiently large aperture; fo that this effential advantage, pecular to the achromatic telescope, will be thereby loft. To this objection M. Jeaurat anfwers, that if, inftead of the refractions which M. D'Alembert ufed in his formula, he had been poffeffed of, and had employed, thofe which are here given, and which he believes to be exact, he would have found that the total destruction of the aberration of refrangibility does not require fuch confiderable curvatures, nor fuch a diminution of the aperture as is apprehended. The remedy to the inconvenience is to be found by increafing the number of the lenfes, and thereby being enabled to diminish the curvatures.

In conformity to the Author's fecond table or fyftem, where' there are conftructions propofed, from a compound object glafs of 2 inches focus, to one of twenty feet, he has executed a telescope, the compound object glafs of which is compofed of four lenfes, formed alternately of crown glafs and Aint glafs, and which has a focus of 5 inches 10 lines. This object-glafs, he obferves, bears an aperture of eighteen lines; whereas the best English achromatic telescopes of fix inches, carry an aperture of only fifteen lines.

As no achromatic object-glaffes have hitherto, as M. Jeaurat fupposes, been conftructed of four or five lenfes, and as it is natural to apprehend that the number of the glaffes may greatly diminish the quantity of light; he obferves that in the foregoing conftruction the lofs of light sustained, in confequence of the number of the lenfes, is more than compenfated for by the diminution of the curvatures, and by the enlargement of the aperture owing to that circumftance. M. Jeaurat terminates his Memoir with fome ufeful practical remarks and directions, relative to the grinding and polifhing of lenfes.

O02

HYDRO

HYDROGRAPHY.

One article only is contained in this Clafs, in which M. Pingré gives an account of fome nautical and aftronomical obfervations made during a voyage to and from the West Indies, in the Ifis, one of the King's frigates, exprefsly fitted out in order to make a trial of the methods propofed for the discovery of the longitude at fea; and more particularly to examine the going of two marine clocks made by M. Berthoud †, which on trial were never found to have erred above two minutes of time in the fpace of fix weeks.

MECHANICS.

This Clafs likewife contains only one Memoir, in which that celebrated Mechanician, M. de Vaucanfon, describes and illuftrates, in feveral plates, the improvements which he has made in the Machine conftructed by him for the ufe of the Royal Silk Manufactory at Aubenas.

The Hiflories of the Arts published this year are, I. That of the Organ Builder, 2d and 3d parts, by Dom. Bedos de Celles, a Benedictine. II. That of the Joiner, part 2d. by the Sieur Ronbo. III. The art of making Indigo, by M. de BeauvaisRafeau. And IV. That of the Embroiderer.

Among the inventions prefented to and approved of by the Academy, we particularly obferve a propofed improvement of Reaumur's Spirit Thermometer, by the Abbé Soumille. On account of its inconvenient and unavoidable bulk, and in order that the degrees on its scale may be enlarged, he has divided it into four smaller diftin&t Thermometers; the first of which has its highest degree marked at the freezing point, and the scale continued 20 degrees below that point. The fcale of the second Thermometer commences below, where the former terminates above; beginning at the freezing point, and proceeding to 20 degrees above it. The third and fourth are conftructed on fimilar principles; fo that the laft carries the scale up to 60°. At the top of each tube there is a refervoir to receive the spirit, whenever it is expofed to a heat greater than that to which its fcale extends. By this ingenious contrivance, fays the Hiftorian of the Academy, the inftrument is rendered more portable, and the intervals on the fcale greatly enlarged.

Without meaning to detract from the undoubted merit of M. Reaumur, in baving been one of the first who constructed a comparable Thermometer; the account of this ingenious device, we think, exhibits a pleasant instance of national partiality in favour of an inconvenient and defective inftrument. To correct one of its many imperfections, an apparatus is to be constructed, confifting of a quadripartite aflemblage of tubes and

+ See the enfuing Article.

balls;

balls; all the purposes of which are, and have long been, effectually answered by a fingle mercurial Thermometer, equally portable, with a fufficient range and extent of intervals, and much more fenfible and accurate.

The remaining contents of the prefent volume are—a Memoir presented by the Academy at Montpelier, on the heat of wine in the firft ftage of fermentation; and the Eloges of the Abbé Nollet, M. Rouelle, and the Earl of Morton.

ART. VIII.

Traité des Horloges marines, &c.-A Treatife of marine Clocks; containing the Theory, the Conftruction, and the Execution of thefe Machines; with the Method of trying them, in order (by means of such Clocks) to rectify the Charts, and determine the Longitude at Sea. With Copper Plates. By M. Ferdinand Berthoud, Clock-Maker to the King, &c. and Fellow of the Royal Society of London. 4to. 590 Pages. Paris. 1773.

MR

R. B. begins his introduction to this work, with explaining the nature of that famous problem which has employed both the scholars and the artifans of feveral ages, viz. to find the longitude at fea. He obferves, that all the methods of folving it may be reduced to two; either by astronomical obfervations, or by machines capable of meafuring time at fea. It is this laft method only that the Author confiders; being by far the most fimple, and what is within the reach of every feaman both to comprehend and practise *.

Mr. B. recounts the feveral attempts made in France, and fays, that himself is the first after Sully (in 1726) that has attempted it again t. Being employed by the Public, Mr. B. thought it his duty to publifh, without referve, all his difcoveries. He has, accordingly, in this work, not only explained the conftruction of his marine clocks, and given drawings of them, but also fet down the dimenfions of every part, the experiments he made, and the reasoning that led him to every determination in a work of twenty years labour and application. Eleven different clocks for measuring time at fea are here defcrib

Mr. B. takes notice that one fingle aftronomical observation is neceffary in every method, namely the finding the altitude of the fun, or a ftar, by Hadley's octant, in order to get the fhip's time. The ufe of that inftrument in taking altitudes at fea, being neceffary for finding the latitude, is now become familiar to every seaman.

+ Mr. Henry Sully, an English watchmaker, fettled at Verfailles in 1718, where he established a manufactory of watches, under the patronage of the regent Duc de Orleans. After two years he returned to England, but foon after went back and established another manufactory at St. Germains. In 1726 he published an account of a longitude clock he had invented, and from which he expected great things; but foon found himself difappointed. He died in 1728.

003

ed,