3 1 8 Models of the Aether. 



at right angles to the plane containing the twist and the 

 direction of the displacement which would exist if the twist 

 were removed ; if the displacement of the medium be repre- 

 sented by F sin pt, and the angular displacement of the twist 

 by w sin pt, the magnitude of the force is proportional to the 

 vector-product of V (in the direction of the displacement) and 

 w (in the direction of the axis of the twist). 



A model of magnetic action may evidently be constructed 

 on the basis of these results. A bar-magnet must be regarded 

 as vibrating tangentially, the direction of vibration being 

 parallel to the axis of the body. A cylindrical body carrying 

 a current will have its surface also vibrating tangentially ; but 

 in this case the direction of vibration will be perpendicular to 

 the axis of the cylinder. A statically electrified body, on the 

 other hand, may, as follows from the same author's earlier work, 

 be regarded as analogous to a body whose surface vibrates in 

 the normal direction. 



We have now discussed models in which the magnetic force 

 is represented as the velocity in a liquid, and others in which 

 it is represented as the displacement in an elastic solid. Some 

 years before the date of Leahy's memoir, George Francis 

 Fitz Gerald (b. 1851, d. 1901)* had instituted a comparison 

 between magnetic force and the velocity in a quasi-elastic 

 solid of the type first devised by MacCullagh.f An analogy 

 is at once evident when it is noticed that the electromagnetic 

 equation 



4?rD = curl H 

 is satisfied identically by the values 



4?rD = curl e, 

 H = e, 



where e denotes any vector; and that, on substituting these 

 values in the other electromagnetic equation, 



- curl (4irc s D/e) - H, 



* Phil. Trans., 1880, p. 691 (presented October, 1878). Fitz Gerald's Scientific 

 Writings, p. 45. t Cf. p. 155. 



