Chap. V.] GEAVITATION AND INERTION. 73 



of the vis -inertia of matter. The force of friction 

 which retards and at length stops the course of the 

 ball rolled along the ground is (as we have before 

 shown) exactly of the same nature as the force which 

 resists that by which the body is supposed to be pro- 

 jected from V. The latter is the force of attraction 

 caused by gravitation drawing towards the earth, and 

 the former is the force of friction, in the ordinary 

 acceptation of the term : but both these forces we 

 have shown to be, in fact, simply eiForts of vis-inertise 

 constantly tending to bring matter to a state of rest. 

 It is then evident that in the case of the ball thrown 

 in the air, or rolled along the ground, as in the case 

 of the moon revolving round the earth, there is no 

 intrinsic difference in the nature of the force which 

 tends to bring them to a state of rest on the surface 

 of the earth. ^ 



^ The ' Laws of Motion ' are more fully discussed in The New 

 Principles of Natural Philosophy^ especially in ' Chapters j^IX., 

 XII., and XVI. ; and partially in the Challenge-Lectures forming 

 Chapters YI. and YII. of that work. 



