218 
Proceedings of Royal Society of Edinburgh. [sess. 
On the Motion produced in an Infinite Elastic Solid by 
the Motion through the Space occupied by it of a 
body acting on it only by Attraction or Repulsion. 
By Lord Kelvin. 
(Read July 16, 1900.) 
§ 1. The title of the present communication describes a pure 
problem of abstract mathematical dynamics, without indication of 
any idea of a physical application. For a merely mathematical 
journal it might be suitable, because the dynamical subject is 
certainly interesting both in itself and in its relation to waves and 
vibrations. My reason for occupying myself with it, and for 
offering it to the Royal Society of Edinburgh, is that it suggests a 
conceivable explanation of the greatest difficulty hitherto presented 
by the undulatory theory of light; the motion of ponderable 
bodies through infinite space occupied by an elastic solid.* 
§ 2. In consideration of the confessed object, and for brevity, 
I shall use the word atom to denote an ideal substance occupying 
a given portion of solid space, and acting on the ether within it 
and around it, according to the old-fashioned eighteenth century 
idea of attraction and repulsion. That is to say, every infinitesimal 
volume A of the atom acts on every infinitesimal volume B of the 
ether with a force in the line PQ joining the centres of these two 
volumes, equal to 
A/(P, PQ) P B (1), 
where p denotes the density of the ether at Q, and / (P, PQ) 
denotes a quantity depending on the position of P and on the 
* The so-called “ electro-magnetic theory of light” does not cut away this 
foundation from the old undulatory theory of light. It adds to that primary 
theory an enormous province of transcendent interest and importance ; it 
demands of us not merely an explanation of all the phenomena of light and 
radiant heat by transverse vibrations of an elastic solid called ether, but also 
the inclusion of electric currents, of the permanent magnetism of steel and 
lodestone, of magnetic force, and of electrostatic force, in a comprehensive 
ethereal dynamics. 
