Chap. IX.] SOLAR AND LUN'AR GRAVITATION. 103 



And, as regards tlie axial rotation of the earth, 

 as that rotation is eastwards, the earth's gravitation 

 tends to carry the ocean eastwards with the surface 

 on which it rests ; but astral gravitation, drawing in 

 the opposite direction, tends to give it a relative mo- 

 tion westwards over the surface of the earth all round 

 the earth. But as the stars draw the water west- 

 wards through, the equatorial regions, where the force 

 of their gravitation, compared with that of the earth, 

 is relatively greater than in the temperate zones, the 

 earth's gravitation draws an equal volume eastwards 

 through the temperate zones, in which the force of 

 its gravitation is relatively greater than in the equa- 

 torial regions : ^ and also as the stars draw the water 

 upwards from the surface of the earth in the equa- 

 torial regions, the earth's gravitation draws an equal 

 volume downwards to the surface of the earth in the 

 temperate zones. From this opposing action of astral 

 and terrestrial gravitation there results the elaborate 

 system of oceanic circulation described in Book II. 

 The circulation there described is quite independent 

 of the mdividual action of the sun and moon, thouo-h 

 the gravitation of those bodies is included in the 

 force of gravitation whose combined action causes 

 that system of circulation. The opposing forces of 

 terrestrial and astral gravitation together represent 

 universal gravitation or the action of vis-inertiae* 



We have, in Book IIL, shown that in the same 



1 See Chapter XXI., Proposition XXVII., Sect. 6. 



