Chap. X.] SOLAR AND LUNAR GRAVITATION. 117 



inerticB, though ocean currents and tides are move- 

 ments of distinctly different natures. 



The ocean currents, as already shown, result 

 from the great cosmical force of gravitation draw- 

 ing in all directions from the centre of the earth, 

 opposing terrestrial gravitation drawing towards 

 the centre of the earth ; and they form a system 

 of circulation within the position in which the 

 ocean is held by gravitation, without tending to 

 cause any changing of the position of the ocean in 

 relation to the surface of the earth ; and these ocean 

 currents would exist even if the earth were not 

 affected by the gravitation — or by any other influence 

 — of the other bodies of the solar system : whereas, 

 the tides result from the disturbing influence of the 

 bodies within the solar system, which cause a con- 

 stant changing of the position of the ocean, in accord- 

 ance with the changes in their positions in relation to 

 the surface of the earth ; but the movements which 

 form the tides are undulations or oscillations of the 

 ocean, and not currents, though, where obstructed 

 by coast-lines, they form currents about the coast. 



The current- creating action of vis-inertite is the 

 cause of the currents by which the circulation of the 

 ocean is efl*ected, and must form the basis of any 

 effective investigation of the tides ; for the study 

 of the tides is, in fact, a branch of the study of 

 vis-inertige, and it is not possible to trace out and ex- 

 plain the tidal movements of the ocean without the 



