DISCOVERY OF THE LAWS OF MOTION. 323 



Circular Motion alone to be naturally uniform, and retains the distinc- 

 tion between Natural and Violent Motion. In the Dialogues on Me- 

 chanics, 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 impedimento ; jam constat ex his quce fusius alibi dicta sunt, 

 illius motum equabilem et perpetuum super ipso piano futurum esse, 

 si planum in infinitum extendatur." " Conceive a movable body upon 

 a horizontal plane, and suppose all obstacles to motion to be removed ; 

 it is then manifest, from what has been said more at large in another 

 place, that the body's motion will be uniform and perpetual upon the 

 plane, if the plane be indefinitely extended." His pupil Borelli, in 1667 

 (in the treatise De Vi Percussionis), states the proposition generally, 

 that " Velocity is, by its nature, uniform and perpetual ;" and this 

 opinion appears to have been, at that time, generally diffused, as we 

 find evidence in Wallis and others. It is commonly said that Descartes 

 was the first to state this generally. His Principia were published in 

 1644 ; but his proofs of this First Law of Motion are rather of a 

 theological than of a mechanical kind. His reason for this Law is, 3 

 " the immutability and simplicity of the operation by which God pre- 

 serves motion in matter. For he only preserves it precisely as it is in 

 that moment in which he preserves it, taking no account of that 

 which may have been previously." Reasoning of this abstract and a 

 priori kind, though it may be urged in favor of true opinions after 

 they have been inductively established, is almost equally capable of 

 being called in on the side of error, as we have seen in the case of 

 Aristotle's philosophy. We ought not, however, to forget that the 

 reference to these abstract and a priori principles is an indication of 

 the absolute universality and necessity which we look for in complete 

 Sciences, and a result of those faculties by which such Science is 

 rendered possible, and suitable to man's intellectual nature. 



The induction by which the First Law of Motion is established, con- 

 sists, as induction consists in all cases, in conceiving clearly the Law, 

 and in perceiving the subordination of Facts to it. But the Law speaks 

 of bodies not acted upon by any external force, — a case which never 

 occurs in fact ; and the difficulty of the step consisted in bringing all 

 the common cases in which motion is gradually extinguished, under 

 the notion of the action of a retarding force. In order to do this, 



1 Dial. i. p. 40. 3 Princip. p. 34. 



