DISCOVERY OF THE LAWS OF MOTION. 27 



The First Law of Motion asserts that the mo- 

 tion of a body, when left to itself, will not only be 

 uniform, but rectilinear also. This latter part of 

 the law is indeed obvious of itself, as soon as we 

 conceive a body detached from all special reference 

 to external points and objects. Yet, as we have 

 seen, Galileo asserted that the naturally uniform 

 motion of bodies was that which takes place in a 

 circle. Benedetti, however, in 1585, had enter- 

 tained sound notions on this subject. In com- 

 menting on Aristotle's question, why we obtain an 

 advantage in throwing by using a sling, he says 4 , 

 that the body, when whirled round, tends to go 

 on in a straight line. In Galileo's second Dialogue, 

 he makes one of his interlocutors (Simplicio), when 

 appealed to on this subject, after thinking intently 

 for a little while, give the same opinion ; and the 

 principle is, from this time, taken for granted by 

 the authors who treat of the motion of projectiles. 

 Descartes, as might be supposed, gives the same 

 reason for this as for the other part of the law, 

 namely, the immutability of the Deity. 



Sect. 2. Formation and Application of the Notion 

 of Accelerating Force. Laws of Falling Bodies. 



WE have seen how rude and vague were the at- 

 tempts of Aristotle and his followers to obtain a 

 philosophy of bodies falling downwards or thrown 



4 " Corpus vellet recta iter peragere." Spectdationum Liber, 

 p. 160. 



