Page:Aether and Matter, 1900.djvu/49

CHAP. II] points or surfaces, (namely in this case given component of velocity normal to the boundaries) the state of motion instantaneously assumed by it is that one for which the kinetic energy is least, which is easily shown to be the irrotational one in the case of a liquid.

As Sir George Stokes was not disposed to admit that the aether could pass freely through the interstices of material bodies in the manner required by Fresnel's views, and as any other theory of its motion which could be consistent with the fact of astronomical aberration required irrotational flow, an explanation of the limitation to that flow had, he considered, to be found. He pointed out that the existence of tangential stress depending not alone (like viscosity) on relative velocities, but also (like elastic stresses) on relative displacements, would make the flow irrotational; for any deviation from irrotational quality would now be propagated away not by diffusion but by waves of transverse displacement, and the coefficient of the elastic part of the force, and consequently the velocity of this propagation, may be assumed so great that the slightest beginning of rotational motion is immediately shed off and dispersed. This chain of argument, that motion of bodies disturbs the aether, that aberration requires the disturbance to be differentially irrotational, that this can only be explained by the dispersion of incipient rotational disturbance by transverse waves, and further that radiation itself involves transverse undulation, he regards as mutually consistent and self-supporting, and therefore as forming distinct evidence in favour of this view of the constitution of the aether. The coexistence of fluidity on a large scale with perfect elasticity on a small scale he illustrates by the ordinary phenomena of pitch or glue, passing on to a limit through jellies of gradually diminishing consistency until perfect fluidity is reached: the chief difficulty here is (as already mentioned) that absolute mathematical