PHYSICAL SCIENCE. 21 



tation in the variety and refinement of its applications, that 

 new grandeur is given to this great law of nature. We may 

 take one or two examples, among many that offer themselves, 

 from our own planetary system ; where this power is more 

 within our cognisance, both in its simple effects and in those 

 complex perturbations of orbits, which have taxed, but not 

 overcome, the efforts of mathematicians. The first instance 

 one of those familiar to the world for the moment, but 

 speedily forgotten is a discovery made by means of these 

 very perturbations. The movements of Uranus, then (1846) 

 supposed the most remote planet of our system, were found 

 to be disturbed by some external influence not referrible to 

 causes within its orbit, as could be shown ; but due to some 

 material attraction from without. Another planet alone 

 could answer these conditions. Science set itself to work 

 in the persons of two eminent mathematicians, Adams and 

 Leverrier ; the position of the disturbing body was deter- 

 mined by them simultaneously, but independently ; tele- 

 scopes followed their guidance, and Neptune was added to 

 the number of our planets. The method of discovery here 

 has higher interest than the fact itself; though now but one 

 of numerous instances in science, where results can be pre- 

 dicted with hardly less certainty than if attained and present 

 to the senses. 



A second example we may cite, in proof of the exactness, 

 or even delicate 'minuteness, with which modern astronomy 

 pursues the vast objects of its science. The complex irre- 

 gularities of the Moon's motions have long put to test all the 

 resources of analysis, and are scarcely even yet fully sub- 

 mitted to our knowledge. Chiefly, of course, they depend on 

 the relative position and distances of the sun and earth ; 

 and Laplace had shown, not only the secular acceleration of 

 mean motion, produced by the increasing eccentricity of the 

 earth's orbit, but also a small irregularity depending on the 



