V2 ATTRACTION. 



moved, he would be in motion relatively to the 

 boat, but would not change his situation with 

 respect to the banks of the river, and would, in 

 fact, be absolutely at rest. 



But, in strictness, as the earth is in constant 

 motion round its own axis, and also round the sun, 

 all the bodies on its surface can only be considered 

 as relatively at rest, the whole partaking in reality 

 of the motion of the earth. Again, we can only 

 judge of the motion of the earth relatively to the 

 sun, which seems the fixed body; yet it is the 

 opinion of astronomers, that the sun, and perhaps 

 all the stars, are in motion; but we can have 

 no idea of their absolute motion, because we have 

 no knowledge of any heavenly body we can be 

 certain is absolutely at rest, to which we can refer 

 their motions. 



Hence we can consider all the motions with 

 which we are acquainted only as relative; abso- 

 lute motion we have no means of ascertaining. 



The force with which a body moves, or which it 

 would exert upon another body opposed to it, is 

 always in proportion to its velocity multiplied by 

 its weight or quantity of matter. This force is 

 called the momentum of the body; for if two equal 

 bodies move with different velocities, it is evident 

 that their forces, or momenta, are as their velo- 

 cities; and if two bodies move with the same 

 velocity, their momenta are as their quantities of 

 matter; therefore, in all cases, their momenta 

 must be as the products of their quantities of mat- 

 ter and their velocities. This rule is the foun- 

 dation of mechanics. 



ATTRACTION. 



By attraction, we mean the tendency that bodies 

 have to approach each other 



