The phenomenon of aberration may be reconciled with the undulatory theory of light, as I have already shown (Phil. Mag., Vol. xxvil. p. 9), without making the violent supposition that the ether passes freely through the earth in its motion round the sun, but supposing, on the contrary, that the ether close to the surface of the earth is at rest relatively to the earth. This explanation requires us to suppose the motion of the ether to be such, that the expression usually denoted by udx + vdy + wdz is an exact differential. It becomes an interesting question to inquire on what physical properties of the ether this sort of motion can be explained. Is it sufficient to consider the ether as an ordinary fluid, or must we have recourse to some property which does not exist in ordinary fluids, or, to speak more correctly, the existence of which has not been made manifest in such fluids by any phenomenon hitherto observed? I have already attempted to offer an explanation on the latter supposition (Phil. Mag., Vol. XXIX. p. 6).

In my paper last referred to, I have expressed my belief that the motion for which udx + &c. is an exact differential, which would take place if the ether were like an ordinary fluid, would be unstable ; I now propose to prove the same mathematically, though by an indirect method.