1899-1900.] Lord Kelvin on the Motion in an Elastic Solid. 233 
of rigid matter of the same bulk as our atom, moving with the 
same velocity. Looking now at the definition of k in the beginning 
of § 12, we may put our conclusion in words, thus: — The dis- 
tribution of ethereal density within our ideal spherical atom 
represented by (11) with K = 100, gives rise to kinetic energy of 
the ether within it at any instant, when the atom is moving slowly 
through space filled with ether, equal to ’634 of the kinetic energy 
of motion with the same velocity through ideal void space, of an 
ideal rigid globe of the same hulk as the atom, and the same 
density as the undisturbed density of the ether. Thus if the atom, 
which we are supposing to he a constituent of real ponderable 
matter, has an inertia of its own equal to I per unit of its volume, 
the effective inertia of its motion through space occupied by 
other will he — s 3 (X + ‘634); the diameter of the atom being 
now denoted by s (instead of 2 as hitherto), and the inertia of 
unit hulk of the ether being still (as hitherto) taken as unit of 
inertia. In all that follows we shall suppose I to he very great, 
much greater than IQ 6 ; perhaps greater than 10 12 . 
§ 15. Consider now, as in § 11 above, our atom at rest; 
and the ether moving uniformly in the space around the 
atom, and through the space occupied by the atom, according 
to the curved stream-lines and the varying velocities shown 
in fig. 5. The effective inertia of any portion of the ether 
containing the atom will he greater than the simple inertia of 
an equal volume of the ether by the amount _ s 3, 634. 
This 
follows from the well-known dynamical theorem that the total 
kinetic energy of any moving body or system of bodies is equal to 
the kinetic energy due to the motion of its centre of inertia, plus 
the sum of the kinetic energies of the motions of all its parts 
relative to the centre of inertia. 
§ 16. Suppose now a transparent body — solid, liquid, or 
gaseous — to consist of an assemblage of atoms all of the same 
magnitude and quality as our ideal atom defined in § 2, and with 
I enormously great as described in § 14. The atoms may be all 
