82 THE ADVANCE OF SCIENCE 



its swing, the bob has a certain amount of 

 potential energy ; as it descends it gradu- 

 ally exchanges this for kinetic energy, 

 until at the centre it possesses an equiva- 

 lent amount of kinetic energy ; from this 

 point onwards, it gradually loses kinetic 

 energy as it ascends, until, at the summit 

 of the other half-arc, it has acquired an 

 exactly similar amount of potential en- 

 ergy. Thus, on the whole transaction, 

 nothing is either lost or gained ; the quan- 

 tity of energy is always the same, but it 

 passes from one form into the other. 



To all appearance, the phenomena ex- 

 hibited by the pendulum are not to be 

 accounted for by impact : in fact, it is 

 usually assumed that corresponding phe- 

 nomena would take place if the earth and 

 the pendulum were situated in an abso- 

 lute vacuum, and at any conceivable dis- 

 tance from one another. If this be so, it 

 follows that there must be two totally dif- 

 ferent kinds of causes of motion : the one 



