In the investigation of this fubject, M. DE LA LANDE.compares, with the tables, above a hundred obfervations, made in different places, taking care to afcertain the true motion of Jupiter. Hence he concludes that, if the apfide of the fatellite be put fixty degrees forward, the calculation will agree fufficiently with obfervation, without making ufe of Wargentin's fecond equation.

This memoir was intended for the information of M. De la Place, who propofed calculating the motions of the fatellites: which, we find, he has fince done, and M. De Lambre is preparing the tables for publication.

On the inferior Conjunction of Venus, January 4, 1781. By

the fame.

The utility of exact obfervations of thefe phenomena, in order to determine the place of the planet's aphelion, is well known to aftronomers. M. DE LA LANDE has here given a comparative view of his own obfervations of this conjunction, with thofe of Profeffor Hornfby at Oxford, of the Duke of Marlborough at Blenheim, of M. Bernard at Marfeilles, and of M. Mallet at Geneva. Hence it appears that, on the day of the conjunction, the mean error of our aftronomer's new tables was 22", which is only eight feconds in the heliocentric longitude; and that the mean time of the true conjunction, at Paris, was 2 hours 26' 50", in 9 figns 14° 15′ 39 of true longitude, computed from the mean equinox, and cleared from the two aberrations. M. DE LA LANDE intends, in the next volume of Memoirs, to give an account of the inferior conjunction of the 7th of Auguft 1788, from which, compared with this, he concludes the aphelion of Venus to be 10 figns 8° 30′ for the commencement of the year 1788.

Concerning the mean Mction of Saturn. By the fame.

The five obfervations of Saturn, recorded by Ptolemy, were ufed by M. Caffini and M. Boulliaud, in order to afcertain the mean motion and fecular equation of this planet: but the refults did not agree with each other: this M. DE LA LANDE afcribes to their not having computed the true places of the fun and ftars for the age in which Ptolemy lived. This aftronomer's catalogue of ftars muft, according to our Acade mician, be referred to the year 128 before Chrift: thus, for the longitude of ftars 200 years before our era, feventeen minutes must be fubtracted, and fixty-four minutes added for the year 140 after Chrift: for the fun's place, thirty-one minutes must be deducted in the former, and fifty-nine minutes added in the latter cafe. By applying thefe corrections, he found that the refults agree very nearly with the tables published, in

1789, by M. De Lambre, calculated according to the theory of M. De la Place.

On the Inclination of the Orbit of Saturn. By the fame.

From Dr. Mafkelyne's obfervations, the academician has calculated that this inclination is 2° 29′ 45′′ for the year 1800. Its diminution is about twenty-three feconds in a century.

Obfervations on a Solar Eclipfe, and on the Longitude of Dantzick. By the fame.

The firft eclipfe of the fun that was accurately obferved, and of which the true time was determined by equal altitudes, was that which happened on July 2, 1666; it was observed at Dantzick by Helvelius, and at Paris by Huygens, Robberval, Azout, Frenicle, and Buot. The difference of meridians between these two cities, calculated from their observations, is 1 hour 5' 22", which is only two feconds lefs than what M. Cagnoli computed it from the eclipfe in October 1781. M. Mechain's obfervations in 1777 and 1778 make it 1 hour 5′ 18′′. Concerning the Theory of Saturn's Ring By M. DE LA

PLACE.

I

The univerfal principle of attraction, by which the other phenomena of the heavenly bodies are fo well explained, is here adopted to account for thofe of Saturn's ring. Accordingly, this ingenious academician has applied, to this fubject, his theory of the attraction of fpheroids, published in the memoirs of this academy for the year 1782*. He fuppofes the figure of the ring to be generated by the revolution of an ellipfe, perpendicular to its plane, around the centre of Saturn, which is placed in the prolonged axis of the generating curve: the breadth of the ring he thinks very fmall, when compared with its distance from the centre of the planet; and he is of opinion that it is compofed of feveral rings, which he confiders as irre gular folids of unequal breadth in different parts of their circumference; fo that their centres of gravity, not coinciding. with their centres of magnitude, may be regarded as fo many fatellites, moving round the centre of Saturn, at diftances which depend on the inequality of the parts of each ring. On this ingenious theory, Dr. Herfchel's late difcoveries will probably enable the author to make fome improvement.

Memoir on the Secular Variations of the Orbits of the Planets. By the fame.

Though it be evident from obfervation as well as from theory, that, in confequence of the mutual attractions of the planets, the elements of their orbits are fubject to variations, the value

* See Review, vol. lxxv. p. 502.

of

of these has not yet been ascertained by obfervations, because, till within the last hundred years, none were made with fufficient accuracy. The theory of gravity has been of great use in the investigation of this fubject, as it has enabled us to trace the causes and the laws of thefe variations: but hitherto we have no exact determination of the mafles of thofe planets which have no fatellites this cannot indeed be expected, till time, by rendering the fecular variations more perceptible, fhall have fupplied us with more accurate data. Then aftronomers will be able to trace the alterations which our fyftem has undergone in paft ages, and to predict thofe which it muft futter in future years. In the mean time, from the fecular inequalities of the planetary orbits, two important truths are deducible, which are independent of the maffes of the heavenly bodies; these are, the stability of the planetary fyftem, and the uniformity of its mean motions. To the latter, the inequalities in the mean motion of Jupiter and Saturn appeared to be exceptions: but, when the causes of these were explained, they confirmed it in the most striking manner, and afforded a strong proof of the univerfal influence of the principle of gravity. On the stability of the planetary fyftem, this academician published a memoir in the year 1784, in which he proved that, as the planets all move in the fame direction, in orbits nearly circular, and but little inclined with refpect to each other, the variations of these, in eccentricity and inclination, are confined within. very narrow limits; fo that our fyftem may be confidered as ofcillating about a mean point, from which it varies very little. Of this propofition he has here given an analytical demonftration, for which we must refer to the memoir.

On the Preceffion of the Equinoxes, and the Obliquity of the Ecliptic, deducted from the Obfervations of Hipparchus. By M. LE GENTIL.

M. LE GENTIL maintains that the mean annual preceffion of the equinoxes, as calculated from the obfervations of Hipparchus, amounts to 49" 40" inftead of 50" 20", which is the computation of M. De la Lande and the Abbé De la Caille. The difference is only two-thirds of a fecond; and, trifling as this is in itself, it becomes of ftill lefs confequence, when we reflect that very little dependence can be placed on the accuracy of the obfervations from which it is deduced. Concerning the obliquity of the ecliptic, we have only the author's affertion that, in the age of Hipparchus, it was twenty-four degrees; the arguments in proof of this he promises to give in a future.

memoir.

* See Review, vol. lxxviii. page 608.

Concerning

Concerning the Time when Telefcopes were firft applied to Aftro nomical Inftruments. By M. DE FOUCHY.

Many aftronomers have afcribed the invention of this im provement to M. Azout, and have afferted that M. Picard was the first perfon who obferved the planets and larger ftars by day-light. M. DE FOUCHY attributes both thefe difcoveries to Morin; who, in a conference held in March 1634, with the commiffioners appointed by Cardinal Richelieu, propofed the application of telescopes, in order to compenfate for the fmall radius of the inftruments ufed at fea for obferving the distance between the moon and ftars. In his Aftronomia Reftituta, which work was first published in that year, Morin gives an account of this difcovery; and, in his work on the longitude, he tells us that toward the end of March 1735, he obferved, with a telescope a foot and a half in length, Arcturus and Venus, after fun-rife. His account of this is written in a moft ridiculous and romantic manner; for he tells us that the experiment was fuggefted to him by a meflenger from heaven, who appeared to him as he was gazing at Jupiter and his fatellites. To this memoir is added a Latin letter, dated Nov. 16, 1674, from Picard to Helvelius, on the fuperiority of telescopes to plain fights in altro. nomical inftruments.

Concerning Binocular Telefcopes. By M. LE GENTIL.

This academician is a powerful advocate for thefe inftruments, and recommends them as greatly fuperior to monocular telescopes with respect to a diftinct view of the object. It must be acknowleged that fome aftronomers have rejected them with too much contempt: but their high price, and the difficulty of procuring object glaffes that are exactly alike, will be obstacles. to their being generally ufed. M. LE GENTIL'S binocle, which, by this account, though not achromatic, was a very good one, was made by Father Gaudibert, a Jacobine friar, who is fince dead.

GEOGRAPHY.

On the Geography of Ptolemy. By M. BUACHE.

M. BUACHE is a great admirer of this ancient geographer; and his memoir is little more than a declamatory panegyric, in which, however, we do not obferve any thing that has not been faid before the intention of its author is to maintain, in oppofition to M. D'Anville, that the Niger of Ptolemy is no other than the river of Senegal: but, for the arguments in fupport of this opinion, we must wait till the publication of his next memoir.

Geographical Obfervations on New Britain and New Guinea. By the fame.

This memoir is intended to confute the opinion advanced by Mr. Alexander Dalrymple, and adopted by Captain Forreft, that the land, which Dampier called New Britain, is no other than what Solomon De Mendana named Solomon's Inlands. M. BUACHE endeavours to fhew that New Britain and New Guinea were both difcovered by the Spaniards, about the year 1527, long before Mendana's voyage to Solomon's Inlands, which are confiderably to the eastward of the former countries. In fupport of this opinion, he adduces a chart published by Tattonus, in the year 1600, of which, together with that of Captain Forrest, he has given copies, reduced to the fame scale. On the Measure of a Degree of the Meridian. By M. DE

LA LANDE.

John Fernel, a phyfician, born at Clermont in the diocefe of Amiens, was the firft, fays M. DE LA LANDE, who, with any accuracy, measured a degree of latitude; of which, in the year 1528, he published an account. According to him, the degree contained 68096 paces, of five feet each, or 56746 toifes but it appears that, in 1668, the Paris toife was shortened by five lines; and, if this alteration be taken into the account, Fernel's measure will be found aftonishingly accurate; as, thus reduced, it becomes 57069 toifes, which agrees with the value of a degree between Paris and Amiens, measured in 1756*. Our academician acknowleges that this accuracy must have been, in a great measure, fortuitous; as the method, to which Fernel had recourfe, did not feem to promife much exactness this was, to travel northward, till, by observing the fun's altitude, he knew that he was a degree diftant from Paris, and then, on his return in a carriage to this city, to count the revolutions of the wheel, which was twenty feet in circumference.

Obfervations on the Situation of the Cafpian Sea. By the fame. We have here an account of an observation of a lunar eclipfe, June 30, 1787, made by M. Beauchamp, at Cafbine, on the coaft of the Calpian Sea, from which the difference of meridians, between this place and Paris, appears to he 3 hours 8' 52" or 47° 13'. This confirms the maps of De Life and Buache, and proves thofe that were drawn by Bonne and D'Anville to be, in this particular, erroneous.

From feveral eclipfes of Jupiter's fatellites, obferved by M. Beauchamp, it appears that the difference of meridians between Paris and Ifpahan, is 3 hours 18', or 49' 30'. This gentieman places ancient Babylon in 32° 34 north latitude, and in 2 hours 47′ 30′′ or 41° 52′ 30′′ eaft longitude from Paris.

* Vide L'Atronomie de De la Lunde, § 2651.

APP. REV. VOL. V.

N n

Memoir