Fitzgerald— 0;^ a Model Illustrating Properties of Ether. 413 



on the squares of f—- -^j, &e., i.e. the strain must be propor- 

 tional to these quantities. Now the simple mechanism of elastic 

 bands will not do this by itself, but it is quite possible to arrange 

 mechanism by which it shall be effected ; for I have been able to 

 invent one working by means of a fluid pumped through a net- 

 work of pipes, by the rotation of wheels so arranged that an accu- 

 mulation of fluid in any place tends to stop the rotation. It is 



easy, then, to arrange that such a series of rotations as — ^ = 



dy d% 



shall produce no accumulation. Having shown, then, that the 

 energy of this medium can be expressed in the same form as that 

 of the ether, it follows at once that all the results deduced from 

 this form of the energy can be reproduced on a model. 



I have explained how it illustrates ordinary electric phenomena, 

 and now I wish to show how the same medium that can illustrate 

 these phenomena can also transmit vibrations analogous to light. 

 If any series of the wheels be suddenly rotated, inertia will prevent 

 those in their neighbourhood from being immediately rotated. 

 After a little while, however, the motion will be transmitted to the 

 neighbouring wheels, which will transmit it to their neighbours, 

 and so on, a wave of motion being transmitted through the medium. 

 If we consider the nature of the motion constituting the wave, it 

 will be seen to be directed in the plane of the wave, i. e. the wave 

 will be a wave of transverse vibration. The axes of rotation of the 

 wheels are evidently at right angles to the direction of propagation of 

 the wave, and what I have described as the direction of polarisation 

 of the bands, is also at right angles both to the axes of rotation 

 and to the direction of wave propagation. This is evident when 

 we consider that the difference of rotation of neighbouring wheels 

 is what causes the tightening of one side of a belt and the loosen- 

 ing of the other ; and the direction of polarisation of the bands is 

 the line joining the centres of the tight and loose sides of these 

 polarised bands, and this line is evidently at right angles to the 

 line joining the centres of the two wheels whose difference of rota- 

 tion causes the polarisation while this latter line is evidently the 

 direction of wave propagation. Now, this is exactly analogous to 

 what Maxwell describes as the nature of the propagation of light in 

 the ether. The axes of rotation represent the direction of magnetic 



