298 NINETEENTH CENTURY. PT. in. 



I have tried, however, to follow as far as possible the plan I 

 adopted in the earlier centuries, of mentioning only a few 

 great men whose work you can understand and follow ; and 

 stating on doubtful subjects what is the opinion of those who 

 are best able to judge from the evidence. Therefore you 

 must constantly bear in mind that this last portion of the 

 book cannot be said to contain a history of the science of 

 the nineteenth century, but only an account of a few of the 

 leading discoveries and theories of our times and of the 

 men who made them. 



Advance in Astronomy. The science of Astronomy, 

 in particular, has spread far beyond our power to follow it. 

 We have seen that astronomers, up to the end of the 

 eighteenth century, were always striving to work out the laws 

 which govern the movements of the heavenly bodies. The 

 key to this problem was found by Newton, and the work was 

 so far completed by Laplace and Lagrange as to show that 

 even those planets which seem to have the most irregular 

 orbits are really governed by the force of gravitation. From 

 that time astronomy became an exact science, and men had 

 only to make their calculations with perfect accuracy in 

 order to be able to foretell what was going to happen ; or, if 

 they failed, then they knew there must be some other un- 

 known heavenly body (such as Neptune, p. 302) causing the 

 irregularity. Therefore the science of astronomy in our 

 century has been chiefly occupied in recording, with ever 

 increasing accuracy, the positions of the heavenly bodies 

 sun, moon, planets, and stars ; and it is from the discussion 

 of these observations that Leverrier has been enabled to 

 produce his theories of the planets, and that Hansen, 

 Pontecoulant, Delaunay, and Adams have perfected the 

 theory of the complicated motion of the moon. And side 

 b> side with this has grown up a new study, namely, that 



