82 HISTORY OF MECHANICS. 



reach of any principles at that time known and 

 understood, some of the mathematicians of the 

 day had rightly solved some cases of it, by pro- 

 ceeding as if the question had been to find the 

 Center of Percussion. The Center of Percussion is 

 the point about which the momenta of all the parts 

 of a body balance each other, when it is in motion 

 about any axis, and is stopped by striking against 

 an obstacle placed at that center. Roberval found 

 this point in some easy cases; Descartes also at- 

 tempted the problem ; their rival labours led to an 

 angry controversy: and Descartes was, as in his 

 physical speculations he often was, very presump- 

 tuous, though not more than half right. 



Huyghens was hardly advanced beyond boyhood 

 when Mersenne first proposed this problem; and ? 

 as he says 7 , could see no principle which even 

 offered an opening to the solution, and had thus 

 been repelled at the threshold. When, however, 

 he published his Horologium Oscillator ium in 1673, 

 the fourth part of that work was on the Center of 

 Oscillation or Agitation; and the principle which 

 he then assumed, though not so simple and self- 

 evident as those to which such problems were after- 

 wards referred, was perfectly correct and general, 

 and led to exact solutions in all cases. The reader 

 has already seen repeatedly in the course of this his- 

 tory, complex and derivative principles presenting 

 themselves to men's minds before simple and elemen- 



7 Hor. Oxc. Pref. 



