3 2 Laws of Motion. [Book I. 



turally remain fb, unlcfs difturbed by fome power act- 

 ing upon it j and a body in motion will continue that 

 motion in the fame direction, and with the fame ve- 

 locity, unlefs flopped or impeded by fome external 

 caufe *. 



The firfl part of this propofition is evident from 

 every part of nature, fmce no part or portion of in- 

 animate matter appears capable of giving itfelf any 

 degree of motion. The latter part of the propofition, 

 namely, that a body will continue its motion for ever, 

 unlefs prevented by external force, it is not fo eafy to 

 illuflrate by experiment, fince we are not able to pro- 

 duce any fpecies of motion which is not in fome de- 

 gree counteracted by the force of gravitation, or by' 

 fome refilling medium. The conclufion, however, 

 appears to be fairly drawn, fmce the lefs the obflruc- 

 tion which is oppofed to any body in motion, the lon- 

 ger the motion continues; thus a ball will continue 

 longer in motion on a fmooth than on an uneven fur- 

 face, whence we may reafonably infer, that if all ob- 

 ftacles were completely removed, motion once com- 

 municated would never ceafe f. 



This property of refiflance in matter is termed, in 

 technical language, its vis inerti<e. 



II. The alteration of the flate of any body, whether 

 from reft to motion, or from one degree of motion to 

 another, is always proportional to the force which is 

 imprefled, and in the direction of that force. 



By this law, the degree of force is fuppofed to be 

 meafured by the greatnefs of the body which it can 

 move with a given velocity. Thus a power which 

 could give to a certain body fuch a degree of celerity 



* Pemberton's View, p. 29. 



f See Enfield's Inftit, Pbilofophy, p. xi, 12. 



in 



