OUTLINES OF NATURAL PHILOSOPHY. 



SECT. II. 



FIllST LAW OF MOTION. 



60. A BODY must continue for ever in a state of 

 rest, or in a state of uniform and rectilineal mo- 

 tion, if it be not disturbed by the action of an ex- 

 ternal cause. 



a. If the body is at rest, it must remain at rest ; for if 

 there is no action of another body, there can be no- 

 thing to determine it to move in one direction more 

 than in another. 



1). If the body is in motion, it will continue to move 

 in the same direction ; for there is nothing to de- 

 termine its deflections to be in one line more than 

 in another line. 



c. Lastly, it cannot change its velocity ; for if its velo- 

 city change, that change must be according to some 

 function of the time ; so that if C be the velocity 

 which the body has at any instant, and t the time 

 counted from that instant, V the velocity at the end 

 of the time , the relation between V, C, and t must 

 be expressed thus, V C -{- A t + B t n +, &c. Now, 

 there is no condition involved, in the nature of the 

 case, by which the coefficients A, B, &c. can be 

 determined to be of any one magnitude rather than 

 of any other ; for by the supposition, there is no 

 other body which acts on the given body, and which, 

 by any relation to it, can afford an equation for de- 

 3 termining 



