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As TR O NOMY. Memoir I. On the true Diameter of the Sun. By M. De la

Lande, Memoir II. On the dark Protuberance observed on the Disk of the

Sun, during the Transits of Venus. By the fame. Astronomers have suspected for some time past that the sun's diameter appears to be relatively diminilbed, in proportion as it js viewed through telescopes of a greater magnifying power, This appearance, it is supposed, proceeds from an augmentation of the apparent diameter of the sun, produced by a luminous circle or crown surrounding his real disk, and depending on the aberration of the solar rays; the effect of which aberration is necessarily diminished in long telescopes, because in them the convexity of the object glass is less, and because the image is larger.

After an examination of the different observations of the late Transits of Venus, particularly of the internal contacts at the ingress and egrefs, M. de la Lande has been confirmed in the idea which he had entertained concerning the cause of this optical augmentation of the sun's disk; and upon the whole he concludes that the diameter of the sun, which he had determined, in his astronomical tables, to be 31' 31" in the apogee, ought to be diminished about 6 seconds and ; by which quantity therefore the duration of the Transits of Mercury and Venus over his surface ought to be diminished.

In the second Memoir M. de la Lande attempts to account, by means of this crown of aberration, for the appearance of the protuberance, or dark ligament, as it has been called, that was observed about the times of the internal contacts, in the two late Transits of Venus in 1761 and 1769. It is difficult to give his explanation of this phenomenon, without the figure accompanying it. "Perhaps however the following account of it may afford the Reader fome idea of his solution.

The crown or circle of aberration abovementioned is only an apparent extension of the real circumference of the solar ditk; or, is only a luminous but un/ubftantial ring, formed by rays scattered on all sides from the real limb. When Venus actually touches the true limb of the fun, the intercepts from our eyes the rays which come from this border ; as they are emanacions proceeding from a part of the disk which is intercepted from out view. Thus the whole of this part of the luminous crown ought to appear dark, like the body of Venus ; and this ligament or black protuberance ought to extend as far as the circumference of the adventitious luminous border.' It ought to become smaller in proportion as the segment of the sun's disk, concealed by the body of Venus, becomes less: and as soon as a single point of the sun's real circumference becomes vibble,

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

As to the white luminous ring, observed by some to surround the planet while she was on the fun's disk, M. de la Lande is of opinion that if it was not an optical illusion, or did not proceed from some defect in the telescopes, the cause of it is to be sought for, in the proper atmosphere of Venus. - Without meaning to contest the existence of an atmosphere round Venus, the phenomenon, we still think, may be satisfactorily accounted for, independent of that supposition, by the cause which we fuggested in our 42d volume ; (May 1770, page 399.) where we proposed an experiment that illustrates and confirms our folution. To explain this matter further, we shall add that

after having long and attentively viewed a black circle placed on a light ground, or the dark body of Venus on the sun's disk, those 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 sensible to its impressions than any ocher part of that membrane. Accordingly, on the least designed 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 impressions of the light, reflected from the white ground immediately circumscribing the dark body, and mult consequently convey to the mind the idea of a luminous border furrounding the dark circle. To the foregoing solution it may be objected thas the luminous ring will confiantly appear to every one who tries the experiment, with due perseverance, on paper ; whereas the phenomenon of the luminous border round Venus was not observed by all those who viewed the transit. But this difference may justly be attributed to various circunstances, the principal of which are—that some observers may have viewed the sun's disk through too dark a glafs ;-or may not have kept the eye invariably fixed on the planet a sufficient time to produce the effect; -or lastly, not having expected, they may not have attended to, the phenomenon. MEMOIR III. On tbe Quantity of the Sun's Parallax, as deduced

from the Transit of Venus in 1769. By M. De la Lande. MEMOIRS IV. and V. On the Observation of the Transit 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 Observations of the Tranfit

of Venus in 1769, and of the Consequences which may be deduced from them. By M. Pingré.

Out of the great number of observations of the late Transit of Venus, M. de la Lande, in the third Memoir, fele&s two, App, Rev. Vol. l.

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in each of which both the ingress and egress of Venus, or the two contacts, had been observed. According to the method employed by him, which does not require any precise knowledge of the longitude of the two places, but which depends on the chord defcribed by Venus's pach over the fun's disk, he concludes the sun's horizontal parallax to be about eight seconds, instead of ten, which had been formerly supposed.

In the fourth Memoir, M. Caffini gives a circumftantial account of the late Abbé Chappe's obfervations relative to the sun'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 those of other Ob. servers, by the method abovementioned, concludes that all of them concur in giving the parallax within very narrow limits; so that we may, without any sensible error, fix it at 8 seconds. and {. Employing this parallax, M. de la Lande has calcu. lated the respective distances of all the planets, their diameters, mafles, denfities, and the velocities of falling bodies at their surfaces; and has given, if the expression may be allowed, a plan and elevation of the intire structure of the solar system, in a table which terminates this Memoir.

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

The remaining articles of this Class are, a Memoir of M. Calfini 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 Eclipses; being his eighth Memoir on the subject: some remarks on the longitude of Cape François ; and a supplement to some ancient observations of the longitude of the moon, compared with the tables.

DIOPTRICS.
MEMOIR. A Determination of the Refraclive and Disper five

Powers of Crown Glafs and Flint Glass, &c By M. Jeaurat.

We have in many of the late volumes of our journal fucceffively given an account of the aitempts of the different Members of the Royal Academies of Paris and Berlin, to ascertain the principles, and improve the construction of that excellent invention, the Achromatic Telescope. In this Memoir, M. Jeaurat gives the result of his experiments and calculations on this fubject, in eight different tables, for the advantage of artists who may not be qualified to prosecute those delicate ex

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See Appendix to our 48th vol. page 560.

periments

periments and calculations which this curious and difficult subject requires.

M. Jeaurat proceeds on this principle; that as it is impoffible totally to annihilate both the aberration proceeding from the spherical figure, and that caused by the different refrangibility of the ravs of light; it is of the greatest importance to destroy that particular aberration which is the greateft and the most prejudicial, viz. the aberration of refrangibility. It happens fortunately that by the fame combination of lenses, formed of dif. ferent kinds of glass, and of different and contrary figures, alternately disposed, by which the aberration of refrangibility is destroyed, that of sphericity is at the lame cime diminished. For these and some other reasons he turns his whole attention to annihila:e 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 lenses must be lo considerable, that it will be impossible to give the object glass of a telescope constructed for this purpose a sufficiently lirge aperture; so that this essential advantage, pecuVar to the achromatic telescope, will be thereby loft. To this objection M. Jeaurat answers, that if, instead of the refractions which M. D'Alembert used in his formula, he had been possessed of, and had employed, those which are here given, and which he believes to be exact, he would have found that the total deftruction of the aberration of refrangibility does not require fuch confiderable curvatures, nor such a diminution of the aperture as is apprehended. The remedy to the inconvenience is to be found by increasing the number of the lenses, and thereby being enabled to diminish the curvatures.

In conformity to the Author's second table or system, where there are constructions proposed, from a compound object glass of 2 inches focus, to one of twenty feet, he has executed a telescope, the compound object glass of which is composed of font lenses, formed alternately of crown glass and Aint glass, and which has a focus of 5 inches ro 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 suppoles, been constructed of four or five lenfes, and as it is natural to apprehend that the number of the glasses may greatly diminish the quantity of lighe; he observes that in the foregoing construction the loss of light sustained, in consequence of the number of the lenses, is more than compensated for by the diminution of the curvatures, and by the enlargement of the aperture owing to that circumstance. M. Jeaurat terminates bis Memoir with fome usefut practicat remarks and directions, relative to the grinding and polifhing of lenfes,

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HYDRO

HYDROGRAPHY. One article only is contained in this Class, in which M. Pingré gives an account of some nautical and astronomical obfervations made during a voyage to and from the West Indies, in the Isis, one of the King's frigates, expressly fitted out in order to make a trial of the methods proposed for the discovery of the longitude at sea, and more particularly to examine the going of two marine clocks made by M. Berthoud +, wbich on trial were never found to have erred above two minutes of time in the space of fix weeks.

MECHANICS. This Class likewise contains only one Memoir, in which that celebrated Mechanician, M. de Vaucanlon, describes and illustrates, in several plates, the improvements which he has made in the Machine constructed by him for the use of the Royal Silk Manufaclory 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._ll. That of the Joiner, part 2d. by the Sieur Ronbo. III. The art of making Indigo, by M. de BeauvaisRaseau. And IV. That of the Embroiderer.

Among the inventions presented to and approved of by the Academy, we particularly observe a proposed improvement of Reaumur's Spirit Thermometer, by the Abbé Soumilie. 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 distinct Thermometers; the first of which has its highest degree marked at the freezing point, and the scale continued 20 degrees below that point. The scale 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 constructed on fimilar principles; so that the last carries the scale up to 60°. At the top of each tube there is a reservoir to receive the spirit, whenever it is exposed to a heat greater than that to which its scale extends. By this ingenious contrivance, says the Historian of the Academy, the instrument is rendered more portable, and the intervals on the scale 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 instrument. To correct one of its many imperfections, an apparatus is to be constructed, consisting of a quadripartite assemblage of tubes and

+ See the ensuing Article,

balls;

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