matter. If there is complete connexion, the ether near the earth is relatively stagnant, and negative results are natural. If there is complete independence, the ether is either absolutely stationary or has a velocity-potential, and the negative results are thereby explained.
Ordinary astronomical aberration, and all other phenomena concerned with vision through strata high above the earth, so far as they have been accurately observed, are consistent with complete independence, but not with a viscous drag.
On the other hand, the negative result of Mr. Michelson’s attempt to detect a second-order effect appears only to be consistent with relative stagnation.
A doubtful positive result, supposed to be obtained by Fizeau (§ 26), on a change in the azimuth of the plane of polarization effected by transmission through oblique plates, would, if established, support relative motion between earth and ether.
31. Is it possible for a sphere to move through a fluid without disturbing it rotationally and propagating rotary motion into space?
It is not possible for an ordinary solid moving through an ordinary fluid. Diffusion of motion, or viscosity, is bound to occur.
It is possible for a vortex ring or assemblage of vortex rings, because at their surface there is no slip. It is possible also if tbe sphere be a solidified portion of the fluid, which condenses in front and evaporates behind (as already mentioned).
Professor Stokes seems to say, that though not possible to retain a velocity- potential with any viscosity, yet with some kind of rigidity it may be possible, because deviations from irrotational motion go off into space with the speed of light. If so, the earth might possibly carry some ether with it^ and yet a ray be straight. I do not see any way in which it can abstain from rotationally disturbing the fluid if at the same time it has to carry some with it. Neither, I think, do Mr. Hicks or Mr. Larmor, to whom I wrote.
Lord Kelvin, however (‘Papers,’ vol. iii., p. 436), has invented an “ether” or kinematically rigid incompressible ideal substance, which satisfies electromagnetic equations and magnetic boundary conditions, whose equations of motion are like those of an elastic solid, and which yet permits locomotion of smooth solids filling vesicular hollows in it, and which in general “takes precisely the same motion for any given motion of the boundary as does a frictionless incompressible liquid in the same space showing the same boundary.”
The experiment now to be described proves, I think, that by the motion of ordinary masses of matter the ether is appreciably undisturbed, and raises a presumption in favour of the earth’s motion being equally impotent.
The one thing in the way of the simple doctrine of an ether undisturbed by motion is Michelson’s experiment, viz., the absence of a second-order effect due to terrestrial movement through free ether. This experiment may have to be explained away; perhaps as suggested above (end of § 25).
