Chap.J.] INERTION. 7 



local causes, but requires the study of cosmical laws. 

 These we will therefore take into consideration first. 

 By the force of gravitation drawing towards the 

 centre of the earth, the ocean is held down in the 

 hollows of the earth's undulated surface, and kept 

 from overwhelming what is at present dry land. 

 The level of the ocean is not, however, determined 

 solely by this direct force of the earth's gravitation ; 

 for it is well known that the spherical form which 

 that force tends to cause is changed by the earth's 

 axial rotation to an oblate spheroid. It is therefore 

 evident that the ocean does not partake of the earth's 

 rotation by any spontaneous tendency ; but that it is 

 dragged round by the surface on which it rests. ^ 

 As this is an unchanging influence, unceasingly act- 

 ing in the same manner, its effects naturally claim 

 a careful consideration before attempting to unravel 

 the intricacies of those resulting from ever-changing 

 causes.^ And it is well, before entering on a discus- 



^ In the Treatise on Vis-Inertice the qiiestion of the existence 

 or non-existence of any action of that force in the ocean is treated 

 as a question to be solved ; whereas in this work the oblate 

 spheroidal form of the earth is accepted as a sufficient demonstra- 

 tion of its action in the ocean and on the surface of the earth, 

 leaving only the further amount and nature of that action to be 

 ascertained. 



'^ Not only is the tidal action of the sun and moon an ever- 

 changing influence in respect to any given part of the ocean, but 

 it is also very slight in comparison with the action of the earth's 

 rotation; for the extreme efiect of that tidal action is to cause, 

 over a very limited area, a rise and fall of about 120 feet, where- 

 as the earth's rotation maintains a constant difference of level 



