288 NINETEENTH CENTURY. PT. in. 



same time we omit others who are equally eminent ; and 

 secondly, we must speak of many subjects which are still, as 

 it were, on their trial, and which will not be finally settled 

 till they can be judged dispassionately by future generations. 

 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 

 completed when Laplace and Lagrange showed 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 really 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. 294) causing the 

 irregularity. Therefore, the science of astronomy in our 

 century has been chiefly occupied in discovering new planets, 

 stars, and star- clusters ; at every step giving us new proof 

 that gravitation rules throughout the visible universe. And 



