324 HISTORY OF MECHANICS. 



Hooke and others showed that, by diminishing the obvious resistances, 

 the retardation also became less ; and men were gradually led to a dis- 

 tinct appreciation of the Resistance, Friction, &c., which, in all terres- 

 trial motions, prevent the Law from being evident ; and thus they at 

 last established by experiment a Law which cannot be experimentally 

 exemplified. The natural uniformity of motion was proved by exarnin- 

 ino- all kinds of cases in which motion was not uniform. Men culled 

 the abstract Rule out of the concrete Experiment; although the Rule 

 was, in every case, mixed with other Rules, and each Rule could be 

 collected from the Experiment only by supposing the others known. 

 The perfect simplicity which we necessarily seek for in a law of nature, 

 enables us to disentangle the complexity which this combination ap- 

 pears at first sight to occasion. 



The First Law of Motion asserts that the motion 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 entertained sound notions on this subject. In comment- 

 ing 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 attempts of Aristotle 

 and his followers to obtain a philosophy of bodies falling downwards 

 or thrown in any direction. If the First Law of Motion had been 

 clearly known, it would then, perhaps, have been seen that the way to 

 understand and analyze the motion of any body, is to consider the 



4 



Corpus vellet recta iter peragcre." Speculutionum Liber, p. 160. 



