460 The Electric and Luminiferous Medium. [Dec. 7, 



gular momentum of the gyrostat and to the absolute angular dis- 

 placement of the medium. An ideal gyrostatic cell has been imagined 

 by Lord Kelvin in which the coexistence of pairs of gyrostats spin- 

 ning on parallel axles in opposite directions cancels the first of these 

 forcives, thus leaving only a static forcive of a purely elastic rota- 

 tional type. The conception of an aether which is sketched by him on 

 this basis* is essentially the same as the one we have here employed, 

 with the exception that the elemental angular velocity of the medium 

 is taken to represent magnetic force, and in consequence the medium 

 fails to give an accounb of electric force and its static and kinetic 

 manifestations. A gyrostatic cell of this kind has internal freedom, 

 and therefore free vibration periods of its own; it is necessary to 

 imagine that these periods are very small compared with the periods 

 of the light waves transmitted through the medium, in order to avoid 

 partial absorption. The, propagation of waves in this aether, having 

 periods of the same order as the periods of these free vibrations, 

 would of course be a phenomenon of an altogether different kind, in- 

 volving diffusion through the medium of energy of disturbed motion 

 of the gyrostats within the cells. 



Lord Kelvin has shown that a fluid medium, in turbulent motion 

 owing to Torticity distributed throughout it, would also possess rota- 

 tional elasticity provided we could be assured of its permanence. 

 Professor Gr. F. Fitzgerald proposes to realise such a medium by 

 means of a distribution of continuous vortex filaments, interlacing in 

 all directions; if the vorticities of the filaments in an element of 

 volume are directed indifferently in all directions, the motional part 

 of the kinetic forcive on the element, which depends on the first 

 power of the vorticity, will be null, while the positional part depend- 

 ing on the square of the vorticity will remain, just as in the gyrostatic 

 medium above considered. The atoms may now be imagined to con- 

 sist of vortex rings making their way among these vortex filaments, 

 and thus a very graphic and suggestive scheme is obtained; the 

 question of stability is however here all-important. No ultimate 

 theory can be final ; and schemes of the kind discussed in this paper 

 may not inaptly be compared to structural formulas in modern 

 chemistry; they bind together phenomena that would otherwise 

 have to be taken as disconnected, though they are themselves pro- 

 visional and may in time be replaced by more perfect representations. 



The electric interpretation of MacCullagh's optical equations, 

 which forms the basis of this paper, was first stated so far as I know 

 by Professor G. F. FitzGerald, ' Phil. Trans.,' 1880. I have recently 

 learned, from a reference in Mr. Grlazebrook's Address, British 

 Association, 1893, that an electric development of Lord Kelvin's 

 * Lord Kelvin (Sir W. Thomson), < Collected Papers,' rol. 3, 1890, pp. 436 

 472. 



