Harwell. 275 



magnetic field produced by electric currents is compared to the 

 flow of an incompressible fluid, so that the magnetic vector is 

 represented by the fluid velocity, then the electric currents 

 correspond to the vortex-filaments in the fluid. This analogy 

 correlates many theorems in hydrodynamics and electricity ; 

 for instance, the theorem that a re-entrant vortex-filament is 

 equivalent to a uniform distribution of doublets over any 

 surface bounded by it, corresponds to Ampere's theorem of the 

 equivalence of electric currents and magnetic shells. 



In his memoir of 1855, Maxwell had not attempted to 

 construct a mechanical model of electrodynamic actions, but 

 had expressed his intention of doing so. " By a careful study," 

 he wrote,* " of the laws of elastic solids, and of the motions of 

 viscous fluids, I hope to discover a method of forming a 

 mechanical conception of this electrotonic state adapted to 

 general reasoning " ; and in a foot-note he referred to the effort 

 which Thomson had already made in this direction. Six years 

 elapsed, however, before anything further on the subject was 

 published. In the meantime, Maxwell became Professor of 

 Natural Philosophy in King's College, London a position in 

 which he had opportunities of personal contact with Faraday, 

 whom he had long reverenced. Faraday had now concluded 

 the Experimental Researches, and was living in retirement at 

 Hampton Court ; but his thoughts frequently recurred to the 

 great problem which he had brought so near to solution. It 

 appears from his note-book that in 1857f he was speculating 

 whether the velocity of propagation of magnetic action is of the 

 same order as that of light, and whether it is affected by the 

 susceptibility to induction of the bodies through which the 

 action is transmitted. 



The answer to this question was furnished in 1861-2, 

 when Maxwell fulfilled his promise of devising a mechanical 

 conception of the electromagnetic field.* 



* Maxwell's Scientific Papers, i, p. 188. 

 t Bence Jones's Life of Faraday ii, p. 379. 



I Phil. Mag. xxi (1861), pp. 161, 281, 338; xxiii (1862), pp. 12, 85; 

 Maxwell's Scientific Papers, i, p. 451. 



T 2 



