1 1 8 On AttraSfion and Repul/ton.^ 



*' Philofophy ; it is eafy to conceive that in a pen- 

 *' dulum there muft be feme one point, on each 

 ^' fide of which, the momenta of the feveral parts 

 *' of the pendulum will be equal, or in which 

 *' the whole gravity might be collec"ted without 

 " altering the time of its vibrations. This 

 '* point, which is called the center of ofcil- 

 " lation, is different from the center of gra- 

 *' vity : for if a plane, perpendicular to the ftring 

 ** of the pendulum, be conceived to pafs through 

 *' the center of the ball, bifedling it, the velo- 

 " city of the lower half, and confequently its 

 " momentum, will in vibration be greater than 

 ** that of the upper half: confequently, the cen- 

 " ter of ofcillation muft be lower than the center 

 *' of gravity ; and a plane paffing through the 

 " center of ofcillation will divide the ball into 

 '* two unequal parts, fo that the greater quan- 

 " tity of matter above it (hall compenfate for 

 " the greater velocity below it, and the mo- 

 *' menta on each fide be equal. If the pen- 

 " dulum be an inflexible rod, every where of 

 " equal fize, it is found that the diftance of 

 " the center of ofcillation from the point of 

 *' fufpenfion is two thirds of the length of the 

 " rod. If, whilft a pendulum is in motion, it 

 " meets with an obftacle at its center of ofcilla- 

 ** tion fufficient to ftop it, the whole motion 

 ** of the pendulum will ceafe at once without 

 «' any jarring : for the obftacle refifts equal 



" momenta 



