This is the theory then,--this theory of elasticity as dependent on motion,--which, in combination with the estimate of density, makes the internal energy of the ether so gigantic. For in every cubic millimetre of space we have, according to this view, a mass equivalent to what, if it were matter, we should call a thousand tons, circulating internally, every part of it, with a velocity comparable to the velocity of light, and therefore containing--stored away in that small region of space--an amount of energy of the order 10²⁹ ergs, or, what is the same thing, 3 × 10¹¹ kilowatt centuries; which is otherwise expressible as equal to the energy of a million horse-power station working continuously for forty million years. SUMMARISED BRIEF STATEMENTS CONCERNING THE ETHER _(As communicated by the author to the British Association at Leicester, 1907)._ 1. The theory that an electric charge must possess the equivalent of inertia was clearly established by J.J. Thomson in the _Philosophical Magazine_ for April, 1881. 2. The discovery of masses smaller than atoms was made experimentally by J.J. Thomson, and communicated to Section A at Dover in 1899. 3. The thesis that the corpuscles so discovered consisted wholly of electric charges was sustained by many people, and was clinched by the experiments of Kaufmann in 1902. 4. The concentration of the ionic charge, required to give the observed corpuscular inertia, can be easily calculated; and consequently the size of the electric nucleus, or electron, is known. 5. The old perception that a magnetic field is kinetic has been developed by Kelvin, Heaviside, FitzGerald, Hicks, and Larmor, most of whom have treated it as a flow along magnetic lines; though it may also, perhaps equally well, be regarded as a flow perpendicular to them and along the Poynting vector. The former doctrine is sustained by Larmor, as in accordance with the principle of Least Action, and with the absolutely stationary character of the ether as a whole; the latter view appears to be more consistent with the theories of J.J. Thomson. 6. A charge in motion is well known to be surrounded by a magnetic field; and the energy of the motion can be expressed in terms of the energy of this concomitant field,--which again must be accounted as the kinetic energy of ethereous flow. 7. Putting these things together, and considering the ether as essentially incompressible--on the strength of the Cavendish electric experiment, the facts of gravitation, and the general idea of a connecting continuous medium--the author reckons that to deal with the ether dynamically it must be treated as having a density of the order 10¹² grammes per cubic centimetre. (See Appendix 2.)