In solving the problem of the precession of the equi noxes, D'Alembert, and other mathematicians, had ne glected to consider the reaction of the ocean. La Place has determined the effect of this reaction, and has shewn that whatever be the law of the depth of the ocean, and whatever be the figure of the spheroid which it covers, the phenomenon of the precession and nutation will be the same as if the ocean formed a solid mass with this spheroid. The lunar theory is also under great obliga tions to La Place. The 18th equation of longitude in Mason's tables, amounting to 7".7 was an empirical equation, which theory did not recognise, and it was therefore omitted in the calculations made for the Nau tical Alinanack. La Place, however, found that such an inequality did exist, and that it arose from the com pression at the earth's poles, which produces a small change upon the law of its attraction. He also disco vered a new equation of the moon's latitude, amounting to 6".5, arising from the same cause which produces a libration in the lunar orbit in a period of the moon's nodes. The acceleration of the rnoon's mean motion had long perplexed astronomers. It had been ascribed by some, to the resistance which the moon experienced in the medium through which she moved, and La Place himself seems at first to have imagined, that it was owing to the successive transmission of gravity through celestial space. After minute investigation, however, he found that the moon's acceleration was owing to a small change in the eccentricity of the earth's orbit, and that its period is so long, that interval of 2000 years it may be considered as v g uniformly. He estimated this inequality at 11".2 in a century, and found that there was a secular acceleration in the mean ano maly of the moon amounting to 48".16, and of 7".84 in the moon's node. Another equation of the moon, amounting to 11", and Wing a period of 185 years, was also discovered by La Prace, and is introduced into the new lunar tables. After all the labours of Euler and La Grange, the great inequalities of Jupiter remained completely unexplained, which induced astronomers to imagine that they arose from some cause different from gravitation. La Place resumed the investigation, and found from the theory of gravity, that both Saturn and Jupiter were subject to an inequality that had a period of 917 years, amounting at a maximum to 48' 44" in the case of Jupiter, and to 20' 49" in the case of Saturn : These results accord most wonderfully with ancient ob servations, and remove every difficulty which attended the perturbations of these planets. It would be impos sible, in a work like this, to enumerate all the improve ments which physical astronomy owes to La Place. His investigations respecting the earth's figure ; his theory of the satellites of Jupiter and Saturn ; his researches on comets, and astronomical refraction, are to be added to the great achievements which we have already de tailed. These interesting investigations are all con tained in his Traite de Afecanique Ceksse, a work which ranks beside the Principle of Newton, and which will ever be regarded as one of the uoulest monuments of intellectual greatness.
There are few practical astronomers to whom the science has been more indebted, than to the celebrated Dr Maskclyne, who succeeded Dr Bliss as astronomer royal in the year 1765. IIis uninterrupted observations on the sun, moon, planets, and fixed stars, which are regularly published, possess a degree of accuracy which has been hitherto unknown, and have enabled the phy sical astronomer to fix the numerical coefficients of his formula. For the publication of the Nautical Almanack,
and the perfection of the lunar method of firming the longitude at sea, we are in a great measure indebted to this excellent astronomer. His unwearied exertions in this great cause of humanity and- science entitle him to the gratitude of the remotest posterity. The im provement of the lunar tables was also owing in some measure to Dr Maskelyne's exertions. He directed Mr Mason, by precise rules, in the improvement of Mayer's tables; and he pointed out to him the equa tions in Mayer's theory, which, though omitted in his tables, might be ascertained from Bradley's observa tions. His catalogue of fundamental stars, his inge nious prismatic micrometer, and his accurate observa tions on the attraction of Sheilallien, will contribute, along with his other labours, to perpetuate the reputa tion of this great and amiable man.
While physical and practical astronomy were making such rapid advances, a new ara began to be formed in the science by the discoveries of Dr Herschel. Aided by the most powerful telescopes that have ever been constructed, this laborious astronomer has extended our knowledge of the planetary and sidereal system far be yond its former limits. On the 13th March, 1781, he discovered a new planet without the orbit of Saturn, to which he gave the name of the Georgium Sidus; on the 1 lth January, 1787, he discovered the second and fourth satellites, which circulate round this primary planet; and in 1790 and 1794, he detected no fewer than other four satellites revolving round the same body. In 1789, he discovered the sixth and seventh satellites of Saturn, which are the innermost of these secondary planets. He also determined the rotation and figure of Saturn, discovered that it had a double ring, and was marked with several belts parallel to its equator. His observations on the sun, moon, and all the planets, have given us new and interesting information respecting the phenomena of these bodies ; but we must refer an account of his various results, to the chapter which con tains an account of the planetary bodies. By comparing the proper motions of the fixed stars, Dr Herschel has discovered, that the whole system is advancing to the constellation Hercules, or to a point in the heavens where right ascension is 250° 52' 30", and whose north polar distance is 40° 22'. He supposes that this motion is performed round some distant centre, and has endea voured to determine its velocity ; but many ages must elapse before this enquiry can be crowned with success. The theory of the construction of the heavens advanced by this celebrated astronomer is equally new and su blime; he supposes the milky way to be an immense nebula, and the solar system to be placed near one of its sides, and he imagines that each nebula, of which he has observed more than 900, consists of a group of suns with their attendant planets. The ingenious microme ters, and other optical instruments, which he has invent ed; his important discoveries respecting radiant heat; his researches respecting coloured rings, united with the extent and sublimity of his astronomical discove ries, entitle us to consider Dr Herschel as one of the greatest benefactors of science. At the advanced age of 72, this venerable astronomer continues to observe the heavens with the most unwearied assiduity. We ardently hope, that a vigorous old age will enable him to pursue this delightful occupation; and when Nature shall withdraw him from the labours of science, we look forward with confidence to the exertions of a suc cessor who promises to inherit the virtues and the ta lents of his father.