750 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
Throughout a region devoid of elasticity this electromotive pressure d' must be 
constant, and the electric force just outside its boundary must he along the normal; 
in the dielectric S-' must satisfy Laplace’s equation, and so be the potential of an 
ideal superficial distribution of matter ; but the electric force is not now derived from 
a potential, although its curl is derived from the potential d'just specified. 
The phenomena of electrostatics require that this term does not occur in the 
energy; and that may be either (i) because d^jclx + dr^jdy + dijdz is null, and 
the medium so to speak incompressible, or (ii) because A is null, so that the medium 
offers no resistance to laminar compression. But there is, apparently, nothing as yet 
to negative a constitution of the medium approximating extremely close to either of 
these two limiting states for both of which the equations of electrostatics would be 
exact. It has Ijeen shown already that there is absolutely nothing against such a 
supposition in the theory of light. But the experiments of Cavendish in proof of 
the electrostatic law of inverse scjuares, as repeated by Maxwell, may be taken as 
showing tliat tiie ratio of any compressional effect to the rotational part of the 
phenomenon is at any rate excessively minute. A very small compressional term like 
til is might possibly be of advantage in an attempt to include gravitation among 
the manifestations of rethereal activity, a point to be examined later on. It differs 
fundamentally from the compressional term introduced by von Helmholtz into the 
equations of electrodynamics. 
43. We may also apply the variational equation of equilibrium to a volume in the 
interior of the dielectric medium, and therefore subject to surface tractions from the 
surrounding parts. It thus appears that the component surface-tractions in the lether 
in the directions of the axes of co-ordinates are, per unit area lying in the direction 
{/, m, ii), 
nh^g — mcVi, IcVi — na^f, inaAf — lh~g; 
their resultant is tangential, i.e. in the plane of the element; it is equal to the 
component of the electric force in that plane, and is at right angles to tliat conpionent. 
’fliis is the s]iecification of the lethereal stress by which static electromotive 
disturbance is transmitted across a dielectric medium. This stress does not at all 
interfere with tiny irrotational fluid motion wdfich may be going on in the medium, or 
with the normal hydrostatic pressure which regulates such motion. 
Electrostatic Attraction hetivecn Material Bodies. 
44. When two charged bodies are moved relative to each other the total electrical 
energy of strain in the aether is altered ; on the other hand, since the electrical 
displacement (rotation of the aether) is circuital, the charges of the bodies are 
maintained constant. In the absence of viscosity, this loss or gain of energy must be 
due to transference to some other system linked witli the electric system ; it reappears 
