40 
Proceedings of the Royal Society of Edinburgh. [Sess. 
as to the existence of lines of force may be regarded as leading to a new 
mode of representing the luminiferous medium. During the latter half of 
the nineteenth century many attempts were made, especially by British 
physicists, to construct mechanical models to illustrate the properties of the 
aether. These are described in Sir Oliver Lodge’s Modern Views of 
Electricity (1907), and in Professor Whittaker’s History of the Theories of 
Hither and Electricity (1910). The rotatory character of magnetism, 
suggested to W. Thomson (Kelvin) by a study of the magnetic rotation of 
the plane of polarisation of light, was adopted by Maxwell,* and elaborated 
in a series of papers on “ Physical Lines of Force,” in which a theory of 
molecular vortices was applied to magnetic phenomena. He assumed the 
existence in the magnetic field of vortices or eddies, having their axes in 
the direction of the lines of force, and having their direction of rotation 
determined by that of the lines of force. The vortices are separated from 
each other by a single layer of round particles, which are in rolling contact 
with both the vortices which they separate, and, acting as “ idle wheels,” 
allow neighbouring vortices to revolve in the same direction. This 
conception, says Maxwell (p. 346), “ may appear somewhat awkward. . . . 
It is, however, a mode of connection which is mechanically conceivable 
and easily investigated, and it serves to bring out the actual mechanical 
connections between the known electromagnetic phenomena; so that I 
venture to say that anyone who understands the provisional and temporary 
character of this hypothesis, will find himself rather helped than hindered 
by it in his search after the true interpretation of the phenomena.” 
The theory of vortex motion has been applied in constructing models 
of the aether by W. Thomson (Kelvin), f FitzGerald,} and Hicks. § A fluid 
in which portions in rotational and irrotational motion are finely mixed 
together has been termed a vortex-sponge. Laminar disturbances are 
propagated through such a vortex-sponge in the same manner as waves of 
distortion in a homogeneous elastic solid. Kelvin came to the conclusion 
later that hollow vortices are better adapted than ordinary vortex filaments 
for the construction of such models. 
A model of another type is found in Kelvin’s || gyrostatic model of the 
luminiferous working of the aether. This type has been developed and its 
limitations pointed out by Larmor.lf In a lecture delivered before the 
* Maxwell, Phil. Mag., vol. xxi, pp. 161, 281, 338 (1861). 
f Kelvin, Brit. Assoc. Report , p. 473 (1880) ; Math, and Pays. Papers , vol. iv, p. 308. 
| FitzGerald, Scient. Proc. Roy. Dublin Soc. (1885). 
§ Hicks, Brit. Assoc. Report , p. 595 (1895). 
|| Kelvin, Math, and Phys. Papers , vol. iii, p. 466. 
Larmor, JEther and Matter, Appendix E. 
