52 THE OCEAN. [Book IL 



from an eastward to a westward direction occurs at 

 6 P.M., and tliat from a westward to an eastward 

 direction at 6 a.m. 



PART II. 



EFFECTS OF THE MOTION OF THE SOLAR SYSTEM. 



As regards the motion of the Solar System through 

 space, of which a series of astronomical observations, 

 originated by Sir William Herschel, has given us a 

 partial knowledge ; although, reasoning from analogy, 

 any influence resultmg from that motion will pro- 

 bably be among the forces which are subject to 

 cyclical changes, yet, considering the enormous periods 

 embraced in those cycles, the action of such forces 

 may, as far as our present purposes are concerned, be 

 regarded as fixed and constant at all points of the 

 surface of the earth. 



We have not, however, in this case, a knowledge 

 of the motion from which to deduce effects, but the 

 motion itself must be demonstrated by induction 

 from its effects, if, reasoning from analogy, the 

 existence of effects consistent only with some as 

 yet unknown motion be demonstrated. And also it 

 must be observed that any motion which may thus 

 be demonstrated by the action of vis -inertias does 

 not necessarily indicate simply a motion of the Solar 

 System ; for, as far as the evidence deducible from 



