SCIENCE IN THE LAST HALF CENTURY. 81 



down to the ceutor of tbe arc, so it must needs be used up by the pas- 

 sage of tbe bob upward from tbe center of tbe arc to tbe summit of tbe 

 left-baud balf swing. Hence, at this point, the bob comes to a momen- 

 tary rest. The last fraction of kinetic energy is just neutralized by the 

 action of the attractive forces, and tbe bob has only potential energy 

 equal to that with which it started. Bo that tbe sum of the phenomena 

 may be stated thus : At tbe summit of either half-arc of its swing, the 

 bob has a certain amount of potential energy : and as it descends it 

 gradually exchanges this for kinetic energy, until at the center it pos- 

 sesses an equivalent amount of kinetic energy ; from this point on- 

 wards, it gradually loses kinetic energy as it ascends, until, at the 

 summit of the other half-arc, it has required an exactly similar amount 

 of potential energy. Thus, on tbe whole transaction, nothing is either 

 lost or gained ; the quantity of energy is always the same, but it passes 

 from one form into the other. 



To all apjiearance, the phenomena exhibited by the pendulum are 

 not to be accounted for by impact ; in fact, it is usually assumed that cor- 

 responding phenomena would take place if tbe earth and the i>endulum 

 were situated in an absolute vacuum, and at any conceivable distance 

 from one another. If this be so, it follows that there must be two 

 totally ditiereut kinds of causes of motion ; the one impact — a vera cama, 

 of which, to all appearance, we have constant experience ; the other, 

 attractive or repulsive "force" — a metaphysical entity- which is phys- 

 ically inconceivable. Xewton expressly repudiated the notion of the 

 existence of attractive forces, in the sense in which that term is ordi- 

 narily understood ; and he refused to put forward any hypothesis as to 

 tbe physical cause of the so-called " attraction of gravitation," As a 

 general rule, his successors have been content to accept the doctrine of 

 attractive and repulsive forces, without troubling themselves about the 

 philosophical difficulties which it involv^es. But this has not always 

 been the case ; and the attempt of Le Sage, in the last century, to show 

 that tbe phenomena of attraction and repulsion are susceptible of ex- 

 planation by bis hypothesis of bombardment by ultra-mundane par- 

 ticles, whether tenable or not, has the great merit of being an attempt 

 to get rid of the dual conception of the causes of motion which has 

 hitherto prevailed. On this hypothesis, tbe hammering of tbe ultra- 

 mundane corpuscles on the bob confers its kinetic energy on the one 

 hand, and takes it away on tbe other ; and tbe state of potential energy 

 means tbe condition of the bob dnring the instant at which tbe energy 

 conferred by tbe hammering during tbe one half-arc has just been ex- 

 hausted by the hammering during the other half-arc. It seems safe 

 to look forward to the time when the conception of attractive and re- 

 pulsive forces, having served its purpose as a useful piece of scientific 

 scaftblding, will be replaced by tbe deduction of the phenomena known 

 cis attraction and repulsion, froni the general laws of motion, 

 e. Mis. 600 



