QQ THE OCEAN. [Book HI. 



CHAPTER V. 



THAT MATTEE, BY VIRTUE OF AN INHERENT FORCE 

 OF INERTION, ENDEAVOURS CONSTANTLY TO BRING 

 ITSELF TO A STATE OF REST. 



We have seen that, as far as their action in the ocean 

 is concerned, vis-inertise and gravitation are conver- 

 tible terms, according as a tendency to move to or 

 from any given power of gravitation is referred to. 

 It must not, however, be assumed from this that they 

 are really in their abstract nature identical. 



Newton defines vis-inertise as ' an innate force of 

 matter, or power of resistmg, by which every body, 

 as much as in it lies, endeavours to persevere in its 

 present state, whether it be of rest, or of moving 

 uniformly forwards in a right line.' ^ 



Now, that any bodies have an innate tendency to 

 move uniformly forwards in a right line is mere 

 assumption. And this has been assumed to be so 

 because it was found that the sun's gravitation is 

 constantly tending to draw the planets towards it ; 

 but as they do not approach the sun, it is clear that 

 an equal force tends to carry them in the opposite 



^ The Princi2na, Book I., Definition III. 



