2 MOTION IN STRAIGHT LINES. 



the nature of centrifugal force as defined by Sir 

 Isaac Newton, is perhaps greater and of more im- 

 portance than may be at first sight apparent. 



Descartes based his philosophy on the assumption 

 that the orbs of heaven have a naturally inherent 

 property of motion, by which they tend to move con- 

 stantly in straight lines. 



Hobbes reasserted this assumption, and from it 

 deduced the now generally accepted theory of the 

 axial rotation of the moon, which is, in fact, in- 

 separable from the Cartesian theory of motion in 

 right lines ; for the denial of the axial rotation of the 

 moon necessitates the rejection of Descartes' theory 

 of motion in straight lines. 



The great triumphs of Newton are independent of 

 the acceptance or rejection of this corner-stone of 

 Descartes' philosophy. He nevertheless, in a some- 

 what modified manner, accepted it; and this in a 

 manner less truly philosophical than did Descartes. 

 Newton assumed the motion of the earth to result 

 from a push or impulse given in a straight line, from 

 which it is constantly deflected by the force of gravi- 

 tation drawing towards the sun. And this original 

 push or impulse, which, in fact, becomes an inherent 

 power in the earth, tending to carry it onwards in a 

 straight line, is the force which, according to the 

 Newtonian philosophy, counteracts the attraction 

 proceeding from solar gravitation, and prevents the 

 earth from being drawn towards the sun. According 



