Chap. X.] SOLAR AND LUNAR GRAVITATION. 113 



regular rise and fall along each meridian, as well as 

 along the parallels of latitude. 



It will be observed that the action here described 

 harmonises with the views arrived at in Book III., 

 through the arguments in that and the preceding 

 Books, for though we have here shown that the tide 

 is carried eastwards in the equatorial regions by the 

 earth's rotation, it is just as true, according to the 

 action of vis -inertia described in the foregoing Books, 

 that, as in the equatorial regions the tides vibrate 

 westwards, following the force which raises them, 

 where its action is relatively greater than the earth's 

 gravitation, at the same time, in the temperate zones, 

 where the earth's gravitation is relatively greater 

 than that of the foreign force, it causes an equal 

 vibration eastwards, and so maintains the equilibrium 

 of the ocean ; and therefore, though in the ocean as 

 it exists the tides cannot sweep round the earth with 

 the regular cyclonic vibrations above described, their 

 course will be determined by the action of vis-inertiae 

 — for the same counteracting forces are brought into 

 play in each instance, preserving the equilibrium of 

 the ocean. If the latter surrounded the earth as an 

 unbroken expanse of water, then the tides would 

 simply be excrescences upon the ocean, through which 

 the circulation described in Book II. would run its 

 course without being affected by them. And in the 

 actual ocean, though the fragmentary tides will be 

 sources of disturbance, as they are broken and dcr 



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