58 HISTORY OF MECHANICS. 



the parts of a body, as, for instance, cutting, bruis- 

 ing, and breaking, with its effect in moving the 

 whole. 



The former difficulty had been seen with some 

 clearness by Galileo himself. In a posthumous ad- 

 dition to his Mechanical Dialogues, he says, " there 

 are two kinds of resistance in a moveable body, one 

 internal, as when we say it is more difficult to lift a 

 weight of a thousand pounds than a weight of a 

 hundred ; another respecting space, as when we say 

 that it requires more force to throw a stone one 

 hundred paces than fifty 4 ." Reasoning upon this 

 difference, he comes to the conclusion that "the 

 Momentum of percussion is infinite, since there is 

 no resistance, however great, which is not overcome 

 by a force of percussion, however small 5 ." He fur- 

 ther explains this by observing that the resistance 

 to percussion must occupy some portion of time, 

 although this portion may be insensible. This cor- 

 rect mode of removing the apparent incongruity of 

 continuous and instantaneous force, was a material 

 step in the solution of the problem. 



The Laws of the mutual Impact of bodies were 

 erroneously given by Descartes in his Principia; 

 and appear to have been first correctly stated by 

 Wren, Wallis, and Huyghens, who about the same 

 time (1669) sent papers to the Royal Society of 

 London on the subject. In these solutions, we per- 

 ceive that men were gradually coming to apprehend 

 4 Op. iii. 210. * iii. 211. 



