70 THE OCEAN. [Book III. 



from the sun, is said to be the tendency of its vis- 

 inerticT3 to carry it uniformly forwards in a straight 

 line.-^ The argument, as given by Newton in The 

 System of the World, is as follows : — 



^ The first law of motion was invented after tlie discovery of 

 the motion of the earth ; and was invented for the express purpose 

 of explaining the earth's continuous motion. 



No proof of that law can be given by physical phenomena 

 about the earth. 



Newton accepted it as a necessity for the purpose of explaining 

 not only the continuous motion of the earth, but also its resistance 

 to the centripetal force. 



This latter was the piu^pose which made the law most imme- 

 diately requisite to him. The New Principles of Natural Philosophy, 

 p. 235. 



Kepler first asserted the onward motion of the earth to be due 

 to an axial rotation of the sun. But he then knew nothing of 

 the laws even of the direct action of gravitation, and had not the 

 slightest idea of the revolving action of gravitation exerted by the 

 sun. 



"When Newton discovered the laws of gravitation, Kepler's idea 

 was abandoned by every philosopher, as being incompatible with 

 the laws of gravitation as far as they were then understood. 



Newton himself says, regarding this point : ' Though gravity 

 might give the planets a motion of descent towards the sun, 

 either directly or with some little obliquity, yet the transverse 

 motion by which they revolve in their several orbs, required the 

 Divine Arm to impress them according to the tangents of their 

 orbs.'i 



And elsewhere he says : ' I do not know any power in Natvire 

 which would cause this transverse motion without the Divine 

 Arm.' 2 



It was for the purpose of explaining this ceaseless motion of 

 the earth without loss of velocity, that the idea was first conceived 

 of matter having, when once in motion, an inherent tendency to 

 continue perpetually in motion. Idem, p. 309. 



' Letter to Dr. Bentley, dated Cambridge, February 11, 1693. 

 '^ Letter dated January 17, 1693. 



