M. Poinsot on the Percussion of Bodies. 167 



C, in a direction perpendicular to the line OGr. If, therefore, an 

 equal and contrary force were opposed at this point C, or if a 

 fixed obstacle were there presented to the body, all its motion 

 would be destroyed. 



From this it would appear as if this point C were that by 

 means of which the body, turning around the point 0, would 

 strike any obstacle or fixed point it might encounter with the 

 greatest possible force ; hitherto, in fact, this point or centre of 

 percussion has been regarded as that corresponding to the 

 greatest percussion which the body is capable of producing 

 against an obstacle, and it is precisely by this property of maxi- 

 mum percussion that authors most frequently define this point, 

 as may be seen in the Encyclopedie, and in most treatises on 

 Mechanics*. 



It will be seen, however, that the point by means of which 

 the body strikes with the greatest force is not the above-men- 

 tioned ordinary centre of percussion, but a new point T, situated 

 between the same and the centre of gravity G, to which it will 

 be convenient to give a particular name. Oi", should it be 

 deemed advisable to avoid new terms, then, since the point C is 

 in reality nothing more than the centre of the unique impulse 

 with which the body is animated, we might call it the centre of 

 impulsion, and reserve the name centre of percussion for the new 

 point T to which it is really appropriate, since T is the point of 

 greatest percussion of the body. But in order not to change 

 denominations already accepted, I shall simply refer to the point 

 T as the centre of maximum percussion. With respect to its 

 precise position in the body, we shall find that its distance from 

 the centre of rotation is the geometrical mean between the 



* la the Encyclopedie ou dictionnaire raisonnee des Sciences, 1765, 

 compiled imder the direction of d'Alembert, we find the centre of percus- 

 sion thus defined: "C'est le point dans lequel le choc ou I'impulsion d'un 

 corps qui en frappe un autre est la jdus grande qu'il est possible . . . . ou 

 bien, le point dans lequel toute la force de percussion du corps est supposee 

 ramassee." 



In the Dictionnaire des sciences mathematiques pures et appliquees, par 

 une society iV anciens eleves de I'Ecole Polytechnique, 1840, we find, added 

 to the above definition, the following : " C'est le point autour duquel, I'eian 

 des parties (du corps) est balance de chaque cote, de maniere a etre arrete 

 par un obstacle iminuable a ce point, et a y rester sans agir sur le centre 

 de suspension." We will add, that Bjirlow, in his article on Mechanics in 

 the ' Encyclopaidia Mctropolitana,' defines the centre of percussion by this 

 last property solely. It is only as a " partial deduction " that he afterwards 

 adds (on p. 139), "When a j)endulum, vibrating with a given angular 

 velocity, strikes an obstacle, the effect of the impact will be greatest if it 

 be made at the centre of jiercussion ; for, in this case, the obstacle receives 

 the whole revolving motion of the pendulum ; whereas, if the blow be 

 struck in any other point, a part of the motion of the pendulum will be 

 employed in endeavouring to continue the rotation." 



