200] DIELECTEICS AND MAGNETIZABLE BODIES. 387 



In like manner we find 



(6') 



as the equations of equilibrium. 



In order to explain electrical and magnetic forces by means of 

 stresses we must therefore be able to transform the expressions 

 already found for 3, H, Z, into forms involving partial derivatives 

 as above. 



Introducing into the expression for B the value of p from 



i (8 / 8F\ d f 8F\ a / dv 



and transforming the derivatives we obtain 



, 



7r^^ W/ 



=sUHSH 8 



8 80 



The expression now has the required form of a sum of three deri- 

 vatives. If we perform similar transformations on H and Z we 

 shall find that the equations of equilibrium are satisfied by putting 



3F3F 1 



dy 4?r ~ 4 



252 



