236 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



Let US for the moment confine ourselves to the case of electrified 

 bodies, the fact that when these move they have to set some of the 

 ether in motion must affect their apparent mass — for exactly the same 

 reason that the apparent mass of a body is greater when it is im- 

 mersed in water than when it is in a vacuum ; when we move the body 

 through the water we have to set in motion, not merely the body itself, 

 but also some of the water around it, in some cases the increase in the 

 apparent mass of the body due to this cause may be much greater than 

 the mass of the body itself. This is the case, for example with air bub- 

 bles in water which behave as if their mass were many hundred times 

 the mass of the air inclosed in them. In the case of the electrified 

 bodies we may picture to ourselves that the connection between them 

 and the ether around them is established in the following way, we 

 may suppose that the lines of electric force which proceed from these 

 charged bodies and pass through the ether, grip, as it were, some of 

 the ether and carry it along with them as they move; by means of 

 the laws of electricity we can calculate the mass of ether gripped by 

 these lines in any portion of space through which they pass. The 

 results of this calculation can be expressed in a very simple way. 

 Faraday and Maxwell have taught us to look for the seat of the po- 

 tential energy of an electrified system in the space around the system 

 and not in the system itself, each portion of space possessing an 

 amount of this energy for which Maxwell has given a very simple 

 expression. Now, it is remarkable, that if we calculate the mass of 

 the ether gripped by the lines of electric force in any part of the 

 space surrounding the charged bodies, we find that it is exactly pro- 

 portional to the amount of potential energy in that si^ace, and is 

 given by the rule that if this mass were to move with the velocity of 

 light the kinetic energy it would possess would be equal to the elec- 

 trostatic energy in the portion of space for which we are calculating 

 the mass. Thus, the total mass of the ether gripj^ed by an electrical 

 system is proportional to the electrostatic potential energy of that 

 system. Since the ether is only set in motion by the sideways motion 

 of the lines of force and not by their longitudinal motion, the actual 

 mass of the ether set in motion by the electrified bodies will be some- 

 what less than that given by the preceding rule, except in the special 

 case when all the lines of force are moving at right angles to their 

 length. The slight correction for this slipping of the lines of force 

 through the ether does not affect the general character of the effect, 

 and in what follows I shall for the sake of brevity take the mass of 

 the ether set in motion by an electrified system to be proportional to 

 the potential energy of that system. The electrified body has thus 

 associated with it an ethereal or astral body, which it has to carry 

 along with it as it moves and which increases its apparent mass. Now, 

 this piece of the unseen universe which the charged body carries along 



