PHYSICS: J. L ARMOR 
335 
by the electron on itself, as given by Mr. Page's formula: moreover on this 
order of ideas there is no reason assignable why electricity should be aggregated 
into electrons at all, unless other forces and constraints extraneous to the 
electric theory are to be introduced to hold them together. 
On the other hand an aether theory demands immediately the existence of 
electrons as the singularities in its own constitution — like vortex-rings in fluid 
— which determine its activity: and the electrons had already formed an 
essential part of electrodynamic theory before they were detected in the free 
state, and the actual magnitude of their inertia thereby ascertained in 
comparison with the masses of the atoms of the dynamical elements. As being 
thus self-subsisting mobile structures, whose interactions with each other at 
distances great compared with the dimensions of their nuclei are fully known, 
but whose internal constitution and adjacent influence could only be -illus- 
trated and guessed at, their dynamical specification will be most suitably, at any 
rate most hopefully, expressed in terms of their electric charge, their effective 
inertia, and their electrodynamic fields of known forms except inside and close 
up to the nuclei; all else being held in suspense as matter only for provisional 
speculation. As in the case of the simplest exemplar of a theory of a medium, 
the hydrodynamic theory for an ideal perfect fluid containing vortex rings or 
moving solids, this inertia may be aeolotropic: it may even when waves can 
travel be a function of the velocity instead of being constant. But it is to be 
the same whether the system which is its seat be moving with acceleration or 
not. The momentum of the system, of type mv, is not to involve its accelera- 
tion: nor is the energy of translatory motion of the body, of type \m x v l where 
m 1 may be different from m. Otherwise all existing general dynamical theory 
would be dissolved. The production of acceleration is ascribed to applied 
force, which measured by its result as the value of d(mv)/dt. The formula 
for the force required to sustain an assigned varying motion of Mr. Page's 
special model of an electron does not in this way introduce only v and its 
acceleration /: the terms in it which involve the gradients of / have no place 
as yet in this scheme. Some origin extraneous to the electron must be assigned 
to them. On an aether-theory that cause is the radiation shot out from the 
electron while it is in varying motion, which convects somehow mechanical 
momentum away from it into the free state; the backward kick of this momen- 
tum acting on the source from which it is ejected is describable as the back- 
pressure of the radiation, which would of course exert a compensating forward 
kick on any obstacle, however distant, which absorbs it. However we may 
clothe our thought in language of relativity, it would appear that this issuing 
radiation does effectively possess an absolute velocity c, and therefore an 
absolute velocity of its source also is theoretically determinable from observa- 
tions made in relation to it: and this seems to explain how it is that Mr. 
Page's formula for the force on a system entirely isolated can involve the 
acceleration of this system and its velocity also. 
