656 Dr G. Green on a Fluid 



at each point of a solid in motion within it. Unless the 

 component of velocity of fluid at each point tangential 

 to the solid were the same as the tangential velocity of 

 the point of the solid in contact with it, the aether in 

 contact with any solid would in general have a motion 

 relative to the solid. The same writer, however, showed 

 later, in discussing a suggestion put forward by Planck, 

 that the condition that the aether should have no motion 

 relative to the Earth would be fulfilled if the aether in 

 contact with the Earth were compressible in accordance 

 with Boyle's law, and at the same time subject to gravity.. 

 It is remarkable that if these assumptions were correct 

 there would be a condensation of aether around the heavenly 

 bodies such as would account for the deflexion of light from 

 its path as observed at the recent eclipse. 



Whether the above suggested explanation of the difficulty 

 is correct or not, the essential point with respect to Stokes's 

 theory is that it accounts for aberration provided the steady 

 irrotational motion of the aether arising from the translational 

 motion of the Earth is very large in comparison with any 

 other motion of the aether which may depend on the rotational 

 motion of the Earth, and which may exist along with the 

 irrotational motion. Arguing from the conditions .pre- 

 senting themselves in the analogous system which we are 

 considering * ; we can admit at least the possibility that this 

 may be the case. In the analogy, the Sun and planets 

 correspond to bodies in motion, rotational and translational, 

 upon the surface of shallow water, the motion being very 

 slow in comparison with the minimum wave-speed in the 

 fluid. The motion of the fluid which is produced by 

 the motion of the solids would in this case be irrotational 

 except for a very small part depending on the motion 

 of rotation of the solid bodies, and ultimately on the very 

 small degree of viscosity within the fluid. An infinitesimal 

 degree of viscosity within the fluid would give complete 

 fulfilment of the condition that the fluid in contact with 

 each solid would have no motion relative to the solid, while 

 the corresponding rotational motion within the fluid would 

 be negligible in comparison with the irrotational motion of 

 the fluid arising from the motion of translation of the solid.. 

 It thus appears that the fluid motion in the neighbourhood 

 of each of the moving solids fulfils exactly the conditions 

 required to be fulfilled in a fluid aether as indicated by the 



* These conditions resemble closely the conditions contemplated by 

 JStokes in his papers — Phil. Mag. vol. xxix. p. 6 (1846), and vol. xxxiii. 

 p, 343 (1848). 



