212 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
ordinary material rigidity, for example by the creation of a lens-shaped cavity, and 
the material on one side of the breach is twisted round in its plane, and continuity 
is then restored by cementing the two sides together, a model of an electric doublet 
or polar molecule wall be jwoduced, the twist in the medium representing the electric 
displacement and being at a distance expressible as due to two conjugate poles in the 
ordinary manner. Such a doublet is permanent, as above ; it can be displaced into a 
different position, at any distance, as a strain-form, without the medium moving 
along with it ; such displacement is accompanied by an additional strain at each 
point in the medium, namely, that due to the doublet in its new position together 
with a negative doublet in the old one. A series of such doublets arranged trans- 
versely round a linear circuit will represent the integrated effect of an electric 
polarization-current in that circuit ; they will imply irrotational linear displacement 
of the medium round the circuit after the manner of vortex motion, but this will 
now involve elastic stress on account of the rigidity. Thus with an ordinary elastic 
solid medium, the phenomena of dielectrics, including wave-propagation, may be 
kinematically illustrated ; but we can thereby obtain no representation of a single 
isolated electric charge or of a current of conduction, and the laws of optical 
reflexion would be different from the actual ones. This materia.1 illustration will 
clearly extend to the dynamical laws of induction and electromagnetic attraction 
between alternating currents, but only in so far as they are derived from the kinetic 
energy ; the law of static attraction between doublets of this kind would be different 
from the actual electric law. 
6. According to the present scheme the ponderomotive forces acting on matter 
arise from the forces acting on the electrons which it involves ; the application of the 
principle of virtual work to the expression for the strain-energy shoAvs that, for each 
electron at rest, this force is equal to its charge multiplied by the intensity of the 
electric field where it is situated. It has been urged that a model of the rethereal 
electric field cannot be complete, and so must be rejected, unless it exhibits a direct 
mechanism by which the ponderomotive normal traction F^Stt is transmitted across 
the sether from the surface of one conducting region to that of another : but the 
position can be maintained that such a representation Avould transcend the limitations 
belonging to a mechanical model of a process wdiich is in part mechanical and in part 
ultra-mechanical. Indeed if this force AA^ere transmitted in the ordinary elastic sense, 
the transmitting stress Avould liaA^e to be of the nature of a self-balancing Faeaday- 
M.vxavell stress involving the square of the sether-strain instead of its first power, 
and tlius not directly related to elastic propagation. The model above described is 
so to speak made of mther, and ought to represent all the tractions that exist in 
mther, vanishing as they do over the surfice of a conducting region ; but the model 
does not in the ordinary sense represent matter at all, except in so far as the mthereal 
strain-form Avhich constitutes the electron is associated AAuth matter. It therefore 
cannot represent directl}^, after the manner of a stress across a medium, a force 
