322 HISTORY OF MECHANICS. 



CHAPTER II. 



INDUCTIVE EPOCH OF GALILEO. DISCOVERY OF THE LAWS OF 

 MOTION IN SIMPLE CASES. 



Sect. 1. Establishment of the First Law of Motion. 



AFTER mathematicians Lad begun to doubt or reject the authority 

 of Aristotle, they were still some time in coming to the conclu- 

 sion, that the distinction of Natural and Violent Motions was alto- 

 gether untenable ; that the velocity of a body in motion increased or 

 diminished in consequence of the action of extrinsic causes, not of any 

 property of the motion itself; and that the apparently universal 

 fact, of bodies growing slower' and slower, as if by their own disposi- 

 tion, till they finally stopped, from which Motions had been called 

 Violent, arose from the action of external obstacles not immediately 

 obvious, as the friction and the resistance of the air when a ball runs 

 on the ground, and the action of gravity, when it is thrown upwards. 

 But the truth to which they were at last led, was, that such causes 

 would account for all the diminution of velocity which bodies experi- 

 ence when apparently left to themselves ; and that without such causes, 

 the motion of all bodies would go on forever, in a straight line and 

 with a uniform velocity. 



"Who first announced this Law in a general form, it may be difficult 

 to point out ; its exact or approximate truth was necessarily taken for 

 granted in all complete investigations on the subject of the laws of 

 motion of falling bodies, and of bodies projected so as to describe 

 curves. In Galileo's first attempt to solve the problem of falling bodies, 

 he did not carry his analysis back to the notion of force, and therefore 

 this law does not appear. In 1604 he had an erroneous opinion on 

 this subject ; and we do not know when he was led to the true doctrine 

 which he published in his Discorso, in 1638. In his third Dialogue 

 he gives the instance of water in a vessel, for the purpose of showing 

 that circular motion has a tendency to continue. And in his first 

 Dialogue on the Copemican System 1 (published in 1630), he asserts 



i Dial. 1. p. 40. 



