320 Models of the Aether. 



spond to a steady magnetic field), and that it is at the same 

 time endowed with the power (which is requisite for the 

 explanation of electric phenomena) of resisting the rotation of 

 any element of volume.* But when the aether moves irrota- 

 tionally in the fashion which corresponds to a steady magnetic 

 field, each element of volume acquires after a finite time a 

 rotatory displacement from its original orientation, in con- 

 sequence of the motion ; and it might therefore be expected that 

 the quasi-elastic power of resisting rotation would be called 

 into play i.e., that a steady magnetic field would develop 

 electric phenomena.f 



A further objection to all models in which magnetic force 

 corresponds to velocity is that a strong magnetic field, being in 

 such models represented by a steady drift of the aether, might 

 be expected to influence the velocity of propagation of light. 

 The existence of such an effect appears, however, to be disproved 

 by the experiments of Sir Oliver Lodge ; J at any rate, unless it 

 is assumed that the aether has an inertia at least of the same 

 order of magnitude as that of ponderable matter, in which case 

 the motion might be too slow to be measurable. 



Again, the evidence in favour of the rotatory as opposed to 

 the linear character of magnetic phenomena has perhaps, on the 

 whole, been strengthened since Thomson originally based his 

 conclusion on the magnetic rotation of light. This brings us 

 to the consideration of an experimental discovery. 



In 1879 E. H. Hall, at that time a student at Baltimore, 



* Larmor (loc. cit.) suggested the analogy of a liquid filled with magnetic 

 molecules under the action of an external magnetic field. 



It has often heen objected to the mathematical conception of a perfect fluid 

 that it contains no safeguard against slipping between adjacent layers, so that 

 there is no justification for the usual assumption that the motion of <i perfect fluid 

 is continuous. Larmor remarked that a rotational elasticity, such as is attributed 

 to the medium above considered, furnishes precisely such a safeguard ; and that 

 without some property of this kind a continuous frictionless fluid cannot be imagined. 



t Larmor proposed to avoid this by assuming that the rotation which is resisted 

 by an element of volume of the aether is the vector sum of the series of differential 

 rotations which it has experienced. J Phil. Trans, clxxxix (1897), p. 149. 



Am. Jour. Math, ii, p. 287 ; Am. J. Sci. xix, p. 200, and xx, p. 161 ; Phil. 

 Mag. ix, p. 225, and x, p. 301. 



