194 JAMES CLERK MAXWELL 



of an engine connected by the crank and the connect- 

 ing-rod, we recognise that the work done on the 

 piston is transmitted thus to the shaft. So, too, in 

 the electro-magnetic field, the ether forms the con- 

 nection between the various circuits in the field; 

 the forces witli which those circuits act on each other 

 are transmitted from one circuit to another by the 

 stresses set up in the ether. 



To take another instance, consider the electro- 

 static attraction between two charged bodies. Let us 

 suppose the bodies charged by connecting each to 

 the opposite pole of a battery; a current Hows from 

 the battery setting up electric displacement in the 

 space between the bodies, and throwing the ether into 

 a state of strain. As the strain increases the current 

 gets less; the reaction resulting from the strain tends 

 to stop it, until at last this reaction is so great that 

 the current is stopped. When this is the case the 

 wires to the battery may be removed, provided this is 

 done without destroying the insulation of the bodies ; 

 the state of strain will remain and shows itself in the 

 attraction between the balls. 



Looking at the problem in this manner, we are 

 face to face with two great (questions the one, What 

 is the state of strain in the ether which will enable it 

 to produce the observed electro-static attractions and 

 repulsions between charged bodies ? and the other, 

 What is the mechanical structure of the ether which 

 would give rise to such a state of strain as will 

 account for the observed forces ? Maxwell gives one 

 answer to the first question ; it is not the only answer 

 which could be given, but it does account for the 



