

^B Models of the Aether. 325 



the analogue of magnetic force ; and a region in which the masses 

 of the wheels are large corresponds to a region of high magnetic 

 permeability. The indiarubber bands of Fitz Gerald's model 

 correspond to the medium in which Maxwell's vortices were 

 embedded ; and a strain on the bands represents dielectric polari- 

 zation, the line joining the tight and slack sides of any band 

 being the direction of displacement. A body whose specific 

 inductive capacity is large would be represented by a region 

 in which the elasticity of the bands is feeble. Lastly, 

 conduction may be represented by a slipping of the bands 

 on the wheels. 



Such a model is capable of transmitting vibrations analogous 

 to those of light. For if any group of wheels be suddenly set 

 in rotation, those in the neighbourhood will be prevented by 

 their inertia from immediately sharing in the motion; but 

 presently the rotation will be communicated to the adjacent 

 wheels, which will transmit it to their neighbours; and so a 

 wave of motion will be propagated through the medium. The 

 motion constituting the wave is readily seen to be directed in 

 the plane of the wave, i.e. the vibration is transverse. The axes 

 of rotation of the wheels are at right angles to the direction 

 of propagation of the wave, and the direction of polarization of 

 the bands is at right angles to both these directions. 



The elastic bands may be replaced by lines of governor 

 balls :* if this be done, the energy of the system is entirely of 

 the kinetic type.f 



Models of types different from the foregoing have been 

 suggested by the researches of Helmholtz and W. Thomson on 

 vortex-motion. The earliest attempts in this direction, however, 

 were intended to illustrate the properties of ponderable matter 

 rather than of the luminiferous medium. A vortex existing in 

 a perfect fluid preserves its individuality throughout all changes, 



* Fitz Gerald's Scient. Writings, p. 271. 



t It is of course possible to devise models of this class in which the rotation may 

 be interpreted as having the electric instead of the magnetic character. Such a 

 model was proposed by Boltzinann, Vorlesungen iiber Maxwell's Theorie, ii. 



