220. ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The science of mechanics is, in a broad sense, the study of those 

 phenomena which depend upon the mutual actions of bodies in 

 bulk. Thus the study of the behavior of a railway car under the 

 combined action of the pull of the locomotive and the drag of the 

 track belongs to the science of mechanics. The study of the beha- 

 vior of a magnet in the neighborhood of an electric circuit belongs 

 to the science of mechanics. The study of the behavior of two /- 

 electrically-charged bodies belongs to the science of mechanics. 



Simple mechanics is the study of ordinary bodies at rest or in 

 motion, and one of the most important ideas in the science of 

 simple mechanics is the idea offeree ; but the science of mechanics 

 is not concerned with, and it sheds no light upon the question as 

 to the exact physical nature of force. Thus, the science of 

 mechanics is not concerned with the question as to the nature of 

 the action which takes place in a gas causing the gas to exert a 

 force on a piston ; the science of mechanics is not concerned with 

 the question as to the nature of the action which takes place in 

 the material of a stretched spring causing the spring to exert a 

 force ; the science of mechanics is not concerned with the nature 

 of the action between the earth and a heavy weight causing the 

 earth to exert a force on the weight ; the science of mechanics is 

 not concerned with the nature of the action which takes place 

 between two bodies which slide over each other and which leads 

 to the production of the force of friction. It is sufficient for the 

 science of mechanics that these actions are what may be called 

 states of permanency of the respective systems. Thus, to say that - 

 a gas in a given cylinder pushes with a force of 100 pounds on a 

 piston, is to specify a definite result of a definite condition or state 

 of the gas, and it is this definite result that is important rather 

 than the details of the physical action which is taking place in the 

 gas. In fact, the science of mechanics owes its existence to the legiti- 

 macy and usefulness of the idea of force irrespective of the nature 

 of the physical processes upon which force action depends* 



*A very remarkable discussion "On the Scope of Mechanical Explanation and 

 on the Idea of Force" is given in Appendix B, pages 268-288, of Larmor's 

 jEther and Matter, Cambridge, 1900. 



