208 PROGRESS OF SCIENCE. 



presence and number lies in the principle of which the 

 nebular hypothesis gives a satisfactory explanation. (See 

 Laplace.) Be this as it may, however, Bode's law was 

 shown to be sound, though empirical, and Piazzi was the 

 first discoverer of a multitude of small worlds. 



1749 1827. Laplace crowned Newton's scientific edifice 

 by his MECANIQUE CELESTE (1799), in which he enunciated 

 the law of universal gravitation gravity being only a 

 particular feature of it. He showed the law ruling Jupiter's 

 satellites, explained, in collaboration with Lagrange, the 

 cause of the moon presenting always the same hemisphere to 

 us ; and he explained the perturbations of our satellite ; why 

 the moon moves round the earth more and more quickly for 

 a lengthy period, and after that gradually more and more 

 slowly. He calculated the long inequality of Jupiter and 

 Saturn, and enunciated therefrom the hypothesis that the 

 mean of the averages in a certain number of planetary 

 revolutions does not vary a cycle, however, covering 

 necessarily thousands, and even millions, of years. This 

 constitutes his theory of the INVARIABILITY OF THE GREAT 

 AXES of planetary orbits, which was further substantiated by 

 Lagrange's calculations. It applies particularly to Jupiter 

 and Saturn, whose orbital perturbations were very striking, 

 and suggested the possibility of a dislocation of our system. 

 Saturn seemed likely to wander away and escape beyond its 

 limits, whilst Jupiter appeared on the contrary to be coming 

 nearer and nearer to the sun threatening to fall into it in 

 the course of time. Laplace showed that these planets 

 behaved in this eccentric manner, because Jupiter makes 

 2^z revolutions round the sun, whilst Saturn makes one only; 

 but as the points of their conjunction (their nearest approach 

 to each other) vary every time, the planets "must right 

 themselves, and resume in time their normal position," and 

 thus obey the law of gravitation. But Laplace is celebrated 

 in particular for propounding the famous theory which bears 

 his name. After long meditations upon the probable origin 

 of the order which reigns in the universe, induced by a 

 striking suggestion due to the genius of Kant (1724 1804), 

 that suns, stars, planets possibly owe their present form to- 



