338 HISTORY OF MECHANICS. 



The term Momentum had been introduced to express the force of 

 bodies in motion, before it was known what that effect was. Galileo, 

 in his Discorso intomo alle Cose che stanno in su VAcqua, says, that 

 " Momentum is the force, efficacy, or virtue, with which the motion 

 moves and the body moved resists, depending not upon weight onlv. 

 but upon the velocity, inclination, and any other cause of such virtue." 

 When he arrived at more precision in his views, he determined, as we 

 have seen, that, in the same body, the Momentum is proportional to 

 the Velocity ; and, hence it was easily seen that in different bodies it 

 was proportional to the Velocity and Mass jointly. The principle thus 

 enunciated is capable of very extensive application, and, among other 

 consequences, leads to a determination of the results of the mutual 

 Percussion of Bodies. But though Galileo, like others of his prede- 

 cessors and contemporaries, had speculated concerning the problem of 

 Percussion, he did not arrive at any satisfactory conclusion ; and the prob- 

 lem remained for the mathematicians of the next generation to solve. 



We may here notice Descartes and his Laws of Motion, the publica- 

 tion of which is sometimes spoken of as an important event in the his- 

 tory of Mechanics. This is saying far too much. The Principia of 

 Descartes did little for physical science. His assertion of the Laws of 

 Motion, in their most general shape, was perhaps an improvement in 

 form ; but his Third Law is false in substance. Descartes claimed sev- 

 eral of the discoveries of Galileo and others of his contemporaries ; but 

 we cannot assent to such claims, when we find that, as we shall see, 

 he did not understand, or would not apply, the Laws of Motion when 

 he had them before him. If we were to compare Descartes with Gali- 

 leo, we might say, that of the mechanical truths which were easily 

 attainable in the beginning of the seventeenth century, Galileo took 

 hold of as many, and Descartes of as few, as was well possible for a 

 man of genius. 



[2d Ed.] [The following remarks of M. Libri appear to be just. 

 After giving an account of the doctrines put forth on the subject of 

 Astronomy, Mechanics, and other branches of science, by Leonardo 

 da Vinci, Fracastoro, Maurolycus, Commandinus, Benedetti, he adds 

 (Hist, dcs Sciences Mathematiques en Italie, t. iii. p. 131) : "This short 

 analysis is sufficient to show that, at the period at which we are ar- 

 rived, Aristotle no longer reigned unquestioned in the Italian Schools. 

 If we had to write the history of philosophy, we should prove by a 

 multitude of facts that it was the Italians who overthrew the ancient 

 idol of philosophers. Men go on incessantly repeating that the strug- 



