214 
:\IR. J. LARMOR OX A DYNAMICAL THEORY OF 
strain-centres as well as the elastic displacement in the medium : and the theory 
which in the simpler case answers fairly to the description of transmission by contact 
action has features in the wider case to which that name does not so suitably apply.'" 
The strain-centres (that is, the matter) have, in the strict sense of the term, energy of 
iwsition, or i-)Oiential energy, due to their mutual configuration in the mther, which 
can come out as work done by mutual forces between them when that confio-uration 
is altered, which work may be used up either in accumulating other potential energy 
elsewhere, or in increasing the kinetic energy of the matter, which is itself, in whole 
or in part, energy in the aether arising from the movement of the strain-forms across it. 
Discussions as to transmission by contact are not the fundamental ones, as the above 
actual material illustration shows : the single comprehensive basis of dynamics into 
which all such partial modes of explanation and representation must fit and be 
coordinated is the formula of Stationary Action, inclnding, as the particular case w hich 
covers all the domain of steady systems, the law^ that the mutual forces of such a 
system are derived from a single analytical function which is its available potential 
energy. 
The circumstance that no mode of transmission of the mechanical forces, of the 
type of ordinary stress across the sether, can Ire put in evidence, thus does not 
derogate from the sufficiency of the present standpoint. The transmission of 
material traction by an ordinary solid, which is now often taken as the tvpe to 
wdiich all physical action must conform, is merely an undeveloped notion arising from 
experience, which must itself be analysed before it becomes of scientific value : the 
explanation thereof is the quantitative development of the notion from the energy 
function by the method of virtual work in the manner indicated in § 10 infra. This 
orderl}^ development of the laws of action across a distance, from an analytical speci¬ 
fication of a distribution of energy pervading the surrounding space, is the essence of 
the so-called principle of contact action. It is precisely what the present procedure 
carries out, with such generalization as the scope of the problem demands; besides 
attaining a correlation of the whole range of the phenomena, it avoids the antinomies 
of partial theories wdiich accumidate on the rether contradictoiy and unrelated j^ro- 
pertles, and sometimes even save appearances by passing on to the simple funda¬ 
mental medium those complex properties of viscous matter whose real origin is to be 
found in its molecular discreteness. 
An analogous principle applies in the vortex-theory- illustration o£ matter. If we consider rigid 
cores round which the fluid circulates, they are moved about hy the fluid pressui’e : hut if we consider 
vortex-rings, say with vacuous cores, these are mere forms of motion that move across the fluid, and if 
we take them to represent atoms, the interactions between aggregations of atoms cannot be traced by 
means of fluid pressures, but cau only be derived from the analytical character of the function which 
expresses the energy. 
