426 Prof. A. Gray on the Virtual Resistance of 



Thus the statement No. 5, made above, is incomplete and 

 inaccurate without the proviso about a steady state of the 

 aether. It appears to be true, so long as it is a case of the 

 statical exertion of force, that such force can only be exerted 

 by matter; but it appears also to be true that the aether or 

 any other medium capable of transmitting rays is able to 

 experience a force on the boundary between two regions 

 possessing different intensities of vibrational energy (the force 

 per unit area being equal to the difference in the energies per 

 unit volume), provided that this boundary advances at a certain 

 speed appropriate to the medium, viz. the speed at which it 

 transmits that particular kind of vibrational energy, or pro- 

 vided that it retreats at the same rate in a direction opposed 

 to the force, — in this latter case destroying the wave-energy 

 which previously existed in the medium. 



Hence 5 may be generalized thus : — 



5'. A stress occurs either in the space between two material 

 bodies, or beticeen a body and an advancing wave-front, or 

 between an advancing and a retreating wave-front. Its action 

 and reaction are always equal and opposite, and a free tutherial 

 reaction is necessarily transmitted with the speed of light. 



In other words, a stress whose one end terminates in aether 

 is necessarily a growing or decaying (or, in the case of both 

 ends, a shifting) and not a statical stress ; it is doing work at 

 a certain definite rate, and thereby generating or destroying 

 aetherial wave-energy. The mechanical force acts not only 

 at obvious wave-fronts, but at every boundary on one side 

 of which there is electric acceleration, i. e. where electric 

 and magnetic forces coexist ; and its value is V(EH). 

 Thus a boundary between pulsating and inert aether behaves 

 in some respects like a material partition ; it is able on certain 

 conditions to take the place of a reacting body in Newton's 

 third law. This is suggestive in connexion with the view that 

 regards all matter as a variety of aetherial strain and motion. 

 The normal pressure on such a boundary results not in normal 

 but in tangential (electric) yield ; there is probably very little 

 meaning in disentangling cause and effect, otherwise the 

 gyrostatic analogy is suggestive. 



XLIII. The Calculation of the Virtual Resistance of Thin 

 Wires for Rapidly Alternating Currents. By Professor A. 

 Gray, F.R.S. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, 



IN his interesting paper on '' The High-Frequency Induc- 

 tion-Coil," published in the current number of the Philo- 

 sophical Magazine, Mr. W. P. Boynton quotes an expression, 



