Chap. X.] SOLAR AND LUNAR GRAVITATION. 115 



meridian, the vis-inertiae of the ocean carries an 

 exactly equal volume to some other part of that same 

 meridian. And so, also, if the luni-solar forces move 

 a volume of water from any part of any parallel of 

 latitude, the vis-inertia3 of the ocean carries an equal 

 volume of water to some other part of the same 

 parallel of latitude. Thus, therefore, as the tidal un- 

 dulations caused by the luni-solar forces rise on any 

 meridian in the equatorial regions, there is a corre- 

 sponding fall on that same meridian in the temperate 

 zones : and as the tide falls on any meridian in the 

 equatorial regions it rises on the same meridian in 

 the temperate zones. Thus the action of vis-inerti^e 

 resulting from the earth's axial rotation causes the 

 tides to move westwards in the equatorial regions and 

 eastwards in the temperate zones, just in the same 

 manner as that in which it causes the oceanic circula- 

 tion described in Book II. 



This action is the same as regards both the solar 

 and the lunar tides, because it acts upon them accord- 

 ing to their position in the ocean, and is in no way 

 concerned with the cause of their being raised in 

 one part of the ocean rather than in another : it simply 

 determines their movements in relation to the sur- 

 face of the earth, and is not concerned with what 

 the cause of the tide may be. And for just the 

 same reason that any given mass of water which 

 the earth's gravitation tends to hold in any part of 

 the ocean is carried through the oceanic circulation 



I 2 



