

The Aether as an Elastic Solid. 157 



contains the displacement, and is at right angles to the 

 rotation. 



MacCullagh's work was regarded with doubt by his own 

 and the succeeding generation of mathematical physicists, and 

 can scarcely be said to have been properly appreciated until 

 FitzGerald drew attention to it forty years afterwards. But 

 there can be no doubt that MacCullagh really solved the 

 problem of devising a medium whose vibrations, calculated in 

 accordance with the correct laws of dynamics, should have the 

 same properties as the vibrations of light. 



The hesitation which was felt in accepting the rotationally 

 elastic aether arose mainly from the want of any readily 

 conceived example of a body endowed with such a property. 

 This difficulty was removed in 1889 by Sir William Thomson 

 (Lord Kelvin), who designed mechanical models possessed of 

 rotational elasticity. Suppose, for example,* that a structure is 

 formed of spheres, each sphere being in the centre of the 

 tetrahedron formed by its four nearest neighbours. Let each 

 sphere be joined to these four neighbours by rigid bars, which 

 have spherical caps at their ends so as to slide freely on the 

 spheres. Such a structure would, for small deformations, behave 

 like an incompressible perfect fluid. Now attach to each bar a 

 pair of gyroscopically-mounted flywheels, rotating with equal 

 and opposite angular velocities, and having their axes in the line 

 of the bar : a bar thus equipped will require a couple to hold 

 it at rest in any position inclined to its original position, and 

 the structure as a whole will possess that kind of quasi- 

 elasticity which was first imagined by MacCullagh. 



This particular representation is not perfect, since a system 

 of forces would be required to hold the model in equilibrium if 

 it were irrotationally distorted. Lord Kelvin subsequently 

 invented another structure free from this defect. t 



* Comptes Eendus, Sept. 16, 1889 : Kelvin's Math, and Phys. Papers, iii, 

 p. 466. 



tProc. Roy. Soc. Edinb., Mar. 17, 1890: Kelvin's Math, and Phys. Papers, 

 iii, p. 468. 



