148 The State of the Medium in the Electrostatic Field [CH. vi 



am unts fcr ' 8* 

 so that the forces on the two ends have as 

 resultant a force tending to move the ether 

 inwards towards the charge. This tendency 

 is of course balanced by the pressures acting 

 on the curved surface, each of which has a 

 component tending to press the ether inside 

 the frustum away from the charge. 



Since R p a) p = R q w q , the former is the greater, 



FlG - 



167. A more complex example is afforded 

 by two equal point charges, of which the lines of force are shewn in 

 fig. 50. 



FIG. 50. 



The lines of force on either charge fall thickest on the side furthest 

 removed from the other charge, so that their resultant action on the charges 

 amounts to a traction on the surface of each tending to drag it away from 

 the other, and this traction appears to us as a repulsion between the bodies. 



We can examine the matter in a different way by considering the action 

 and reaction across the two sides of the plane which bisects the line joining 

 the two charges. No lines of force cross this plane, which is accordingly 

 made up entirely of the side walls of tubes of force. Thus there is a pressure 



R* 



'- per unit area acting across this plane at every point. The resultant of 



O7T 



all these pressures, after transmission by the ether from the plane to the 

 charges immersed in the ether, appears as a force of repulsion exerted by 

 the charges on one another. 



