PHYSICS OF NINETEENTH CENTURY 161 



locomotion drew all such interests to London or the 

 two Universities, where, for fifty years, learning in 

 England became chiefly concentrated. It is only of 

 recent years that the foundation of provincial Uni- 

 versities has re-established local centres of interest 

 and intercommunication. 



The first line of scientific achievement we have to 



follow into the new era is an extension of old methods 



rather than an invention of new ones. 



Astronomy. __ , _, . . , . . , ,. 



Newton s Pnncipia carried the gravita- 

 tional theory as far as the existing mathematical tools 

 allowed ; but during the years 1773 to 1787 that 

 theory was developed much further by the use of 

 modern mathematics by J. L. Lagrange and Pierre 

 Simon, afterwards created Marquis de Laplace. In 

 Laplace's second great period of activity, 1799-1825, 

 he systematized all such knowledge in his monu- 

 mental treatise, Mecanique C&leste, and popularized 

 the Newtonian philosophy in a smaller book, Syst&me 

 du Monde. 



Almost all the complicated planetary motions 

 were explained broadly by this school of mathematical 

 astronomers, though some details of lunar and planet- 

 ary theory were left to be worked out in future years. 

 It may be said at once that the final test was given 

 to Newton's theory by its use to predict the existence 

 of an unknown planet, thus reversing the methods 

 of Newton, Lagrange and Laplace. The perturbations 

 from its orbit of the planet Uranus were not to be 

 accounted for fully from the action of the other known 

 bodies, and, to explain these irregularities, the influence 

 of a new planet was assumed, and its necessary position 



ii 



