THE ASTRONOMICAL VIEW OF NATURE. 319 



after many able mathematicians and observers had gen- 

 erally investigated the numberless problems contained in 

 the ' Principia,' Laplace published his ' Exposition du 

 Systeme du Monde,' followed in the course of the first 

 quarter of this century by the ' Mecanique celeste ' ;^ and 

 at the close of the present century the most learned 

 astronomer of the age could say that the ' Principia ' 

 still formed the sole foundation of all investigations in 

 that domain.^ 



It is interesting to see how in a simple formula the 

 mathematician is able to condense an almost immeasur- 

 able volume of thought, bringing the theory and the 

 observations of past ages to a focus from which new lines 

 of thought diverge in many directions. Every mathe- 



12. 



The gravita- 

 tion for- 

 mula. 



^ The ' Exposition du Systeme 

 du Monde' appeared, 1796, in 2 

 vols. 8vo : the first and second 

 volume of the ' Mecanique celeste,' 

 1799, 4to; the third, 1802; the 

 fourth, 1805 ; the last, 182.1. Be- 

 fore publishing this work, which 

 has been termed a second edition 

 of the ' Principia,' Laplace had 

 himself during thirty years assisted 

 in dispelling the last doubts as to 

 the sufficiency of the doctrine of 

 universal gravitation to explain all 

 cosmical phenomena ; and he had 

 especially brought the investiga- 

 tions of Clairaut, Euler, d'Alem- 

 bert, Lambert, and Lagrange to a 

 final result by publishing in suc- 

 cessive memoirs between 1773 and 

 1786 the doctrine of "the stability 

 of the system of the universe,"' 

 based upon the invariability of the 

 major axes and the periods of re- 

 volution of the planetary orbits. 

 He and his predecessors also ex- 

 tended the solution of the problem 

 " to find the orbit of two bodies, 

 acting under the law of mutual 



gravitation," which was given by 

 Newton in such a way that the 

 action of one or more third (dis- 

 turbing) bodies could be taken into 

 account, dealing thus with the case 

 of nature, which had in the first 

 instance presented itself in treating 

 of the complex motion of the moon. 

 Laplace himself, who in number- 

 less passages of his works re- 

 curs to the discoveries of Newton, 

 announced the object of the 'Me- 

 canique celeste ' to be the treat- 

 ment of astronomy "as a great 

 problem of mechanics, from which 

 it was important to banish as much 

 as possible all empiricism," and to 

 perfect it so as " to borrow from 

 observation only the most indis- 

 pensable data" ('M^c. eel.,' vol. i. 

 introd. ) 



- The late Professor Rudolf Wolf 

 of Zurich, whose ' Haudbuch der 

 Astronomic, ihrer Geschichte und 

 Litteratur,' 2 vols., 1890-93, as well 

 as his earlier ' Geschichte der As- 

 tronomic,' Miinchen, 1877, 1 warmly 

 1 recommend. 



