208 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
stress by media sensibly continuous were however originally formulated in connexion 
with the observed properties of elastic matter ; and the growth of general theories of 
stress-action was throughout checked and vivified by comparison with those pro¬ 
perties. It was thus natural in the first instance to examine whether a restriction to 
the material type of elastic medium forms an obstacle in framing a theory of the 
aether ; but when that restriction has been found to offer insuperable difficulties it 
seems to be equally natural to discard it. Especially is this the case when the scheme 
of properties which specifies an available medium turns out to be intrinsically simpler 
than the one which specifies ordinary isotropic elastic matter treated as continuous. 
A medium, in order to be available at all, must transmit actions across it in time ; 
therefore there must be postulated for it the property of inertia,—of the same kind 
as ordinary matter possesses, for there can hardly be a more general kind,—and also 
the property of elasticity or statical resistance to change either of position or of form. 
In ordinary matter the elasticity has reference solely to deformation; while in the 
constitution here assumed for the aether there is perfect fluidity as regards form, but 
elastic resistance to rotational displacement.* This latter is in various ways formally 
the simpler scheme ; elasticity depending on rotation is geometrically simpler and 
more absolute than elasticity depending on change of shape ; and moreover no 
phenomenon has been discovered which would allow us to assume that the property 
of elasticity of volume, which necessarily exists in any molecularly constituted 
medium such as matter, is present in the aether at all. The objection that rotational 
elasticity postulates absolute directions in space need hardly have weight when it is 
considered that a definite space, or spacial framework fixed or moving, to which 
motion is referred, is a necessary part of any dynamical theory. The other funda¬ 
mental query, whether such a scheme as the one here sketched could be consistent 
with itself, has perhaps been most convincingly removed by Lord Kelvin’s actual 
specification of a gyrostatic cellular structure constituted of ordinary matter, which 
has to a large extent these very properties; although the deduction of the whole 
scheme of relations from the single formula of Least Action, in its ordinary form in 
which the number of independent variables is not unnaturally increased, includes its 
ultimate logical justification in this respect. 
* I find that the rotational a?ther of MacCullagh, which was advanced by him in the form of an 
abstract dynamical system (for reasons similar to those that prompted Maxwell to finally place his 
mechanism of the electric field on an abstract basis) Avas adopted by Rankine in 1850, and expounded 
with full and clear realization of the elastic peculiarities of a rotational medium : by him also the 
important advantage for physical explanation, which arises from its fluid character, AA'as first emphasized. 
Of. Miscellaneous Scientific Papers, pp. 63, 160. In Rankixe’s special and peculiar imagery, the inthcr 
was however a polar medmm or system (as contrasted with a body) made uja of polarized nuclei 
(Cf. Part I., §§ 37-8) whose vortical atmospheres, where such exist, constitute material atoms. The 
supposed necessity of having the vibration at right angles to the plane of polarization also misled him 
to the introduction of complications into the optical theory, such as molotropic inertia, and to deviations 
from MacCullagh’s rigorous scheme. 
