MECHANICS. 143 



p. 133., Sec. VINCE DTI the Principles of Progres- 

 sive and Rotatory Motion. Phil. Trans. 1780, 

 p. 546., &c. 



Rotation on a Movedble Axis. 



. When the impulse communicated to a bo- 

 dy is in a line passing through its centre of gravi- 

 ty, all the points of the body move forward with 

 the same velocity, and in lines parallel to the di- 

 rection of the impulse communicated. But when 

 the direction of the impulse communicated does 

 not pass through the centre of gravity, the body 

 acquires a rotation on an axis, and also a progres- 

 sive motion, by which its centre of gravity is car- 

 ried forward in the same straight line, and with 

 the same velocity, as if the direction of the impulse 

 had passed through the centre of gravity. 



The progressive and rotatory motion are independent 

 of one another, each being the same as if the other 

 had no existence. 



This is a consequence of the general law, That the 

 quantity of motion estimated in a given direction 

 is not affected by the action of the bodies on one 

 another. The revolution of a body on its axb 

 arises from an action of this kind, and can neither 

 increase nor diminish the progressive motion of the 

 whole mass. 



2 The 



