Mr. G. G. Stokes on the Aberration of Light. 7 



motion of the solid be progressive and not oscillatory. This 

 appears to be due to two causes; first, the motion considered 

 would probably be unstable in the part of the fluid behind the 

 solid ; and secondly, a tangential force is called into play by 

 the sliding of one portion of fluid along another ; and this force 

 is altogether neglected in the common equations of hydrody- 

 namics, from which equations the motion considered is de- 

 duced. If, instead of supposing the solid to move continue 

 ously, we supposed it first to be in motion for a very small 

 interval of time, then to be at rest for another equal inter- 

 val, then to be in motion for a third interval equal to the 

 former, and so on alternately, theoretically the fluid ought to 

 be at rest at the expiration of the first, third, &c. intervals, 

 but practically a very slight motion would remain at the end 

 of the first interval, would last through the second and third, 

 and would be combined with a slight motion of the same kind, 

 which would have been left at the end of the third interval, 

 even if the fluid immediately before the commencement of it 

 had been at rest ; and the accumulation of these small motions 

 would soon become sensible. 



Let us now return to the aether. We know that the trans- 

 versal vibrations constituting light are propagated with a ve- 

 locity about 10,000 times as great as the velocity of the earth; 

 and Mr. Green has shown that the velocity of propagation 

 of normal vibrations is in all probability incomparably greater 

 than that of transversal vibrations (Cambridge Philosophical 

 Transactions, vol. vii. p. 2). Consequently, in considering the 

 motion of the aether due to the motion of the earth, we may 

 regard the aether as perfectly incompressible. To explain 

 dynamically the phaenomena of light, it seems necessary to 

 suppose the motion of the aether subject to the same laws as 

 the motion of an elastic solid. If the views which 1 have 

 explained at the end of a paper On the Friction of Fluids, 

 &c. (Cambridge Philosophical Transactions, vol. viii. part 3) 

 be correct, it is only for extremely small vibratory motions 

 that this is the case, while if the motion be progressive, or not 

 very small, the aether will behave like an ordinary fluid. Ac- 

 cording to these views, therefore, the earth will set the aether 

 in motion in the same way as a solid would set an ordinary 

 incompressible fluid in motion. 



Instead of supposing the earth to move continuously, let us 

 first suppose it to move discontinuously, in the same manner 

 as the solid considered above, the aether being at rest just 

 before the commencement of the first small interval of time. 

 By what precedes, the jether will move during the first inter- 

 val in the same, or nearly the same, manner as an incompres- 



