145. The ether, in fact, must be distorted as well as displaced by matter passing through it; but any distortion of the nature of a shear, such as would give rise in water to vortex-motion accompanied by friction (the whole energy being thus ultimately frittered down into heat), would in the ether be handed on at once, as vibratory motion, with the velocity of light. Thus vortex-motion of the ether may be conceived to be impossible, simply in consequence of the minuteness of its density in comparison with the great tangential force called into play by a shear; and a body moving in it with a velocity not so great as that of light would thus not have eddies in its wake, as in an ordinary fluid, but, on the contrary, would be a source of radiation, even although there may have been no heating either of the body or of the medium it is displacing, paradoxical as this result may appear. In this connection it is hardly possible to avoid quoting Milton—though there may be a suspicion of something analogous to a pun:— ‘The grinding sword with discontinuous wound Passed through him—but the ethereal substance closed Not long divisible.’ 146. Sir William Thomson has endeavoured to obtain at least an inferior limit to the density of the ether in planetary space. His method is based upon the measurements by Pouillet and Herschel of the whole amount of radiant energy received from the sun by a given amount of terrestrial surface in a given time, and upon an assumption that the extreme amplitude of distortion of the ether in any radiation is small compared with the length of a wave. In this way he finds that, as a cubic mile of the ether near the earth contains about 12,000 foot-pounds of radiant solar energy, the mass of the ether in that cubic mile must be at least 1/1,000,000,000 of a pound.[48] To show that this is not by any means a surprisingly small quantity he compares it with the mass of a cubic mile of air at a distance of only a few radii from the earth’s surface (supposing that the atmosphere extends so far; which, by the way, the recent calculations of the velocities of the particles of a gas render exceedingly improbable). This, he finds, will be probably represented by a fraction of a pound having unit for a numerator and 329 places of figures in the denominator!!!