PT. i. Astronomy and Geology compared. 15 



and noted with extraordinary accuracy. Within the 

 last century two planets of first-class magnitude and 

 importance (Uranus and Neptune) have been dis- 

 covered. A numerous body of asteroids have been 

 found occupying a large space in our system be- 

 tween Mars and Jupiter. The appearance of any 

 comet within the circumference of our system 

 is immediately perceived, and its course watched 

 and measured. All the motions of the planetary 

 bodies and of their satellites are distinctly visible to 

 us through these great and powerful telescopes. 

 Their changes of position and their revolutions 

 round the sun can be traced with the minutest 

 accuracy, and all this array of facts can thus be 

 marshalled in confirmation of the truth of that great 

 solar system which it is the glory of modern 

 astronomy to have discovered. All this can be 

 established by pursuing the second method that of 

 reasoning on the evidence of facts ; but Astronomy 

 can be supported on that still sounder and more 

 certain basis of pure mathematical demonstration. 

 The ' Principia ' of Newton and the ' Analysis ' of 

 Laplace draw their proofs from the sources of pure 

 mathematics. It is this combination of the results 

 of these two methods which gives to Astronomy, at 



