24 HISTORY OF MECHANICS. 



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 sub- 

 ject of the laws of motion of falling bodies, and of 

 bodies projected so as to describe curves. In Ga- 

 lileo's first attempt to solve the problem of falling 

 bodies, he did not carry his analysis t 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 Coper- 

 nican System 1 (published in 1630), he asserts Cir- 

 cular Motion alone to be naturally uniform, and 

 retains the distinction between Natural and Violent 

 Motion. In the Dialogues on Mechanics, however, 

 published in 1638, but written apparently at an 

 earlier period, in treating of Projectiles 2 , he asserts 

 the true Law. " Mobile super planum horizontale 

 projectum mente concipio omni secluso impedi- 

 mento; jam constat ex his qua9 fusius alibi dicta 

 sunt, illius motum equabilem et perpetuum super 

 ipso piano futurum esse, si planum in infinitum 

 extendatur." "Conceive a moveable body upon 

 a horizontal plane, and suppose all obstacles to 

 1 Dial. i. p. 40. 3 p. 141. 



