Models of the Aether. 327 



motion are finely mixed together, so that on a large scale the 

 mass is homogeneous, having within any sensible volume an 

 equal amount of vortex-motion in all directions. To a fluid 

 having such a type of motion he gave the name vortex-sponge. 



TiveTyears later, Fitzgerald*" discussed the suitability of the 

 vortex-sponge as a model of the aether. Since vorticity in a 

 perfect fluid cannot be created or destroyed, the modification 

 of the system which is to be analogous to an electric field must 

 be a polarized state of the vortex motion, and light must be 

 represented by a communication of this polarized motion from 

 one part of the medium to another. Many distinct types of 

 polarization may readily be imagined : for instance, if the 

 turbulent motion were constituted of vortex-rings, these might 

 be in motion parallel to definite lines or planes ; or if it were 

 constituted of long vortex filaments, the filaments might be 

 bent spirally about axes parallel to a given direction. The 

 energy of any polarized state of vortex-motion would be greater 

 than that of the unpolarized state; so that if the motion of 

 matter had the effect of reducing the polarization, there would 

 be forces tending to produce that motion. Since the forces due 

 to a small vortex vary inversely as a high power of the distance 

 from it, it seems probable that in the case of two infinite 

 planes, separated by a region of polarized vortex-motion, the 

 forces due to the polarization between the planes would depend 

 on the polarization, but not on the mutual distance of the 

 planes a property which is characteristic of plane distributions 

 whose elements attract according to the Newtonian law. 



It is possible to conceive polarized forms of vortex- motion 

 which are steady so far as the interior of the medium is 

 concerned, but which tend to yield up their energy in producing 

 motion of its boundary a property parallel to that of the 

 aether, which, though itself in equilibrium, tends to move 

 objects immersed in it. 



In the same year Hicksf discussed the possibility of trans- 



* Scient. Proc. Roy. Dublin Soc., 188o Scientific Wntings of FitzGerakl, 

 p. 154. + Brit. Assoc. Rep., 1885, p. 930. 



