492 Prof. V. Bjerknes on 



We liave supposed it to be in a state of stress, but we have 

 not in any way accounted for this stress, or explained how 

 it is maintained. This step, however, seems to me to be an 

 important one, as it explains, by the action of the consecutive 

 parts of the medium, phenomena which were formerly 

 supposed to be only explicable by direct action at a distance. 

 " I have not been able to make the next step, namely, to 

 account by mechanical considerations for these stresses in the 

 dielectric. I therefore leave the theory at this point, merely 

 stating etc." 



After Maxwell nobody has yet been able to make this next 

 step. Some formal improvements have been made, but from 

 the point of view of principle the stress theory has not- 

 advanced beyond the point where Maxwell left it. But still 

 we have at least now the advantage of possessing a remark- 

 ably complete mechanical image of the electrostatic and the 

 magnetic phenomena, worked out by the late Professor C. A. 

 Bjerknes of Christiania*. 



It may therefore be worth while to see how Maxwell's stress 

 theory works in the case of these electroidic phenomena, where 

 we are able to account for everything by mechanical con- 

 siderations- 

 Let us imagine a man, who sees the experiment with the 

 attractions and repulsions of pulsating bodies, but who is not 

 able to see the small pulsations, nor the liquid which pro- 

 pagates the action from the one body to the other. This 

 observer will then be in exactly the same limited condition 

 as to knowledge of the hydrodynamic phenomena before him 

 as the electrician is relatively to the electrical phenomena. 

 The attractions and repulsions of these bodies will make upon 

 him exactly the same impression as the attraction and repulsion 

 of electrified bodies or of magnetic poles. His measurements 

 will bring him to the result, that they follow the law of Coulomb, 

 only with the sign of the force reversed. 



Suppose now this observer to be a Maxwell. He will then 

 suspect, that these actions are not real actions at a distance. 

 He will suspect the presence of a medium, and try to explain 

 the apparent actions at a distance by some stress in this medium . 

 And up to this point he will be perfectly right in his con- 

 clusions. 



But this will no longer be the case if he follows Maxwell 

 further, in attempting to give an explicit solution of the stress 

 problem. He will then consequently arrive at the expression 

 of the Maxwell stress, only with the sign reversed for each of 



* V. Bjerknes, " Vorlesungen Uber hydrodviminische Feriikrafte nack 

 C. A. Bjerknes' Theorie, Leipzig," 1900^02. 



