246 POPULAR LECTURES AND ADDRESSES. 



following specification. The two electromagnets 

 are to be of the same shape and size as the two 

 bodies, and to be placed in the same relative 

 positions, and to consist of infinitely thin layers 

 of electric currents in the surfaces of solids 

 possessing extreme diamagnetic quality in other 

 words, infinitely small permeability. The dis- 

 tribution of electric current on each body may be 

 any whatever which fulfils the condition that 

 the total current across any closed line drawn 

 on the surface once through the aperture is 

 equal to i/4?r of the circulation 1 through the 

 aperture in the hydrokinetic analogue. 



It might be imagined that the action at 

 a distance thus provided for by fluid motion 

 could serve as a foundation for a theory of the 

 equilibrium, and the vibrations, of elastic solids, 

 and the transmission of waves like those of light 



1 The integral of tangential component velocity all round any 

 closed curve, passing once through the aperture, is defined as the 

 "Cyclic constant," or the "circulation" Vortex Motion, 60 a 

 (Trans, ft. S. ., April 29, 1867). It has the same value for all 

 closed curves passing just once through the aperture, and it 

 remains constant through all time whether the solid body be in 

 motion or at rest. 



