AND THE NATURE OF FORCE. 495 



of tlie proper motion, he will be unable to conceive how the impulses 

 can be directed and adjusted, except by an intelligence little short of 

 mii-aculous. 



It is inadmissible to regard the orbits as lying in parallel planes, 

 since a certain amount of heat would be required to separate these 

 planes which would destroy the constant relation between the incre- 

 ments of temperature and expansion. 



There is another way of conceiving the motion of the particles, 

 which might be preferred. Suppose the particles, perfectly elastic 

 and united in pairs as before, attracted by a constant force. If the 

 particles of a pair be separated to a certain distance and left to 

 their mutual action, they will approach each other with gradually 

 accelerated motion, strike and rebound, retrograding through exactly 

 the same degrees of velocity, come to rest at the original distance 

 of separation, then repeat the motion. Thus we have a vibrating 

 element. We may suppose the impulses of the ethereal waves, or of 

 the particles of the heating body, to be such as constantly to renew 

 motion lost by resistance. When the impulses are increased by 

 additional heat, they may be such as to increase the amplitude of the 

 vibrations in the same proportion as the actual energy is increased. 

 On this supposition the average dynamic energy will represent the 

 temperature, and the average amplitude the distance due to potential 

 energy. So far all seems well, but there is a fatal objection to this 

 arrangement, viz., the vibrations, as the heat is increased or dimiii- 

 ished, will not be isochronous, which they must be to agree with 

 observation. They might perhaps be made isochronous by supposing 

 peculiar alterations in the impulses of the ether, but then the incre- 

 ments of potential energy would not correspond with the increments 

 of heat, as they should to account for proportionate expansion. 



The only attractive force which will give isochronous vibrations is 

 one varying with the distance, which is inadmissible for the above 

 reason. 



The above are the simplest arrangements which can be devised on 

 the theory of action at a distance, and we see how they fail. 



If we suppose more particles admitted into a system, or other laws 

 of attraction, we only increase the difficulty of arranging vibrations of 

 the ether, which are required to explain observed phenomena. There 

 is another objection to this theory which seems fatal to it. If I 

 remember aright, it was noticed by Faraday. 



