APPEARANCES OF THE CELESTIAL BODIES. 415 



and his investigations, which were founded on those of Newton, led the 

 way to still further improvements and refinements, which have been since 

 made in succession by Euler, Lagrange, and Laplace. 



LECT. XLIIL ADDITIONAL AUTHORITIES. 



Analytical Investigations on the Theory of Gravitation. Euler, Theoria Motuum 

 ilanet. 4to, Berl. 1744. D'Alembert, Hist, et Mem. 1745, p. 365: Recherches 

 sur v la Precession des Equinoxes, &c. 4to, Paris, 1749 : Recherches sur le Systeme 

 du ^onde, 3 vols. 4to, Paris, 1754-6. Bailly, Essai sur la Theorie des Sat. de Jup. 

 4to, f766. Silvabelle, Ph. Tr. 1754, p. 385. Walmsley on Perturbations, ibid. 

 1756, p. 700; 1761, p. 275. Laplace on the Secular Variations of the Planets, 

 Hist, et Mem. 1772, i. 343, H. 67 ; 1784, p. 1 ; 1787, p. 267 : on the Theory of 

 Jupiter and Saturn, ibid. 1785, p. 33; 1786, p. 201. Lagrange on the Secular 

 Variations of the Nodes and Inclinations, ibid. 1774, p. 97, H. 39 ; 1780, p. 285, 

 H. 38. Dionis de Sejour, Traite Analyt. des Mouvemens Apparens des Corps 

 Celestes, 2 vols. 4to, Paris, 1786-9. Fuss on the True Anomaly N. A. Petr. 1785, 

 iii. 302. Cousin, Ast. Physique, 4to, 1787. Schubert on the Obliquity of the 

 Ecliptic, ibid. 1792, x. 433. Gauss, Theoria Motus Corporum Cselestum, 4to, 

 Hamb. 1809. Plana, Memoirs on the Coeff. of the great Inequality of Jupiter and 

 Saturn, 4to, Turin, 1826-28-29-32. Theorie de la Mouvement de la Lune, 3 vols. 

 4to, Turin, 1832. Airy's Tracts, Camb. 1831. Cauchy, Sur la Mec. Cel. 4to, 

 Lilhog. Lubbock on the Theory of the Moon and Perturbations of the Planets, 

 1833-6. Hansen, Theoria Motus Lunse, 4to, Gotha. 



The number of essays on this subject is so very great, and they are scattered so 

 widely over the surface of all the transactions of the learned societies of Europe, that 

 we can do no more than direct the reader to consult their pages. He will find many 

 valuable memoirs in the introduction to the different observations : in the Effemerides 

 of Cesaris, Hell, &c. ; in Schumacher's Astronomic Nachrichten ; in Crelle's and 

 other Journals. The standard works are Newton's Principia, and the treatises given 

 under Lect. II. at the foot of p. 20. The subject is treated popularly in Airy's Gra- 

 vitation, 12mo, 1834. 



LECTURE XLIV. 



ON THE APPEARANCES OF THE CELESTIAL BODIES. 



WE are next to proceed to examine the sensible effects produced by those 

 motions which we have first considered in their simplest state, and after- 

 wards with regard to their causes and their laws. Many authors have 

 chosen rather to pursue a contrary method, and have attempted to imitate 

 the original and gradual development of the primitive motions from their 

 apparent effects. But no conception is sufficiently clear, and no memory 

 sufficiently strong, to comprehend and retain all these diversified appear- 

 ances with accuracy and facility, unless assisted by some previous idea of 

 the real changes which produce them, or by some temporary hypothesis 

 respecting them, which may have been of use in its day for the better 

 connection of the phenomena, although it does not at present deserve to 

 be employed for a similar purpose, in preference to simpler and better 

 theories, which happen to be historically of a later date. 



