SCIENCE IN THE LAST HALF CENTURY. 81 



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

 sage of the bob upward from the ceuter of the arc to the summit of the 

 h'ft-luuul half 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 the bob has only potential energy 

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

 niay be stated thus : At the 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 euergy. Thus, on the w^hole 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 appearance, 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 the earth and the pendulum 

 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 different kinds of causes of motion; the one impact — a veracausa, 

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

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

 ically inconceivable. Newton 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 

 the 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 

 idiilo-sophical difficulties which it involves. But this has not always 

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

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

 l)lanatiou by his hypothesis of bombardment by ultramundane par- 

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

 to get rid of the dual conce])tion of the causes ot motion which has 

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

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

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

 means the condition of the bob during the instant at which the energy 

 conferred by the hammering during the 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 

 scattblding, will be replaced by the deduction of the phenomena known 

 as attraction and repulsion, from the general laws of motion. 

 H. Mis. GOO 



