THE LAWS OF MOTION. Y5 



festations of each mode of force, independently of 

 the disturbances in its manifestations which other 

 modes of force cause in every actual case. In works 

 on Mechanics, the laws of motion are expressed with 

 out reference to friction and resistance of the medium. 

 Not what motion ever really is, but what it would 

 be if retarding forces were absent, is asserted. If any 

 retarding force is taken into account, then the effect 

 of this retarding force is alone contemplated : neglect 

 ing the other retarding forces. Consider, again, the 

 generalizations of the physicist respecting molecular 

 motion. The law that light varies inversely as the 

 square of the distance, is absolutely true only 

 when the radiation goes on from a point without 

 dimensions, which it never does ; and it also assumes 

 that the rays are perfectly straight, which they cannot 

 be unless the medium, differs from all actual media in 

 being perfectly homogeneous. If the disturbing 

 effects of changes of media are investigated, the 

 formula) expressing the refractions take for granted 

 that the new media entered are homogeneous ; which 

 they never really are. Even when a compound 

 disturbance is allowed for, as when the refraction 

 undergone by light in traversing a medium of in 

 creasing density, like the atmosphere, is calculated, 

 the calculation still supposes conditions that are un 

 naturally simple it supposes that the atmosphere 

 is not pervaded by heterogeneous currents, which 

 it always is. Similarly with the inquiries of tho 



