I ATTRACTION. 



mar onetime* actually measure the distance between them, as when one piece 

 of gbuB is hid on another, in which case a considerable pressure must be ap- 

 plied to bring the surfaces near enough to shew the block spot of Newton's 

 ring*, which indicate* a distance of about a ten thousandth of a millimetre. If, 

 in older to get rid of the idea of action at a distance, we imagine a ma- 

 terial medium through which the action is transmitted, all that we have done 

 is to substitute for a single action at a great distance a series of actions at 

 smaller distances between the parts of the medium, so that we cannot even 

 thus get rid of action at a distance. 



The study of the mutual action between the parts of a material system 

 hat, in modem tiroes, been greatly simplified by the introduction of the idea 

 of the energy of the system. The energy of the system is measured by the 

 amount of work which it can do in overcoming external resistances. It depends 

 on the present configuration and motion of the system, and not on the manner 

 in which the system has acquired that configuration and motion. A complete 

 knowledge of the manner in which the energy of the system depends on its 

 configuration and motion, is sufficient to determine all the forces acting between 

 the parts of the system. For instance, if the system consists of two bodies, 

 and if the energy depends on the distance between them, then if the energy 

 increases when the distance increases, there must be attraction between the 

 bodies, and if the energy diminishes when the distance increases, there must 

 be repulsion between them. In the case of two gravitating masses m and m' 



at a distance r, the part of the energy which depends on r is - . We 



may therefore express the fact that there is attraction between the two bodies 

 by saying that the energy of the system consisting of the two bodies increases 

 when their distance increases. The question, therefore, Why do the two bodies 

 attract each other? may be expressed in a different form. Why does the 

 energy of the system increase when the distance increases ? 



But we must bear in mind that the scientific or science-producing value of 

 the efforts made to answer these old standing questions is not to be measured 

 by the prospect they afford us of ultimately obtaining a solution, but by their 

 effect in stimulating men to a thorough investigation of nature. To propose a 

 scientific question presupposes scientific knowledge, and the questions which 

 exercise men's minds in the present state of science may very likely be such 

 that a little more knowledge would shew us that no answer is possible. The 



