79 



CHAPTER VII. 



THAT THE CURRENT-CREATING ACTION OF VIS-INERTIiE, 

 DESCRIBED IN BOOK II., IS CORROBORATED BY THE 

 MOTIONS OF THE PLANETS. 



That the earth's surface, in some manner, tends to 

 carry the moon eastwards is indicated, as pointed out 

 in the foregoing chapter, by the fact that the action 

 of the moon's gravitation raises a tide which goes 

 round with the moon, whilst the earth's surface rolls 

 on eastwards under them. And, since the moon's 

 gravitation drags westwards on the earth's surface 

 (for it drags the tide westwards with it), that surface 

 must, since the action of gravitation is reciprocal, 

 tend to drag the moon eastwards. This reciprocal 

 action of gravitation is inseparable from the existence 

 of that force which, by Newton's demonstrations, 

 may be said to have been removed from the domain 

 of theory by being incontrovertibly established as a 

 simple fact. And, therefore, the assertion that the 

 earth's surface drags the moon eastwards is not only 

 in accordance with Newton's laws of gravitation, but 



