372 REPOBT — 1893. 



destroyed owing to instability, as seems likely, its elasticity would appear 

 at first sight to depend on angular velocity and not on angular displace- 

 ment, so that it could not have the properties of MacCuUagh's aether.^ 

 Lord Kelvin has recently occupied himself^ with the dynamics of media 

 composed of gyrostats mounted on framework having various degrees 

 of mechanical freedom. It is possible to imagine frames devoid of dis- 

 tortional elasticity and either incompressible or devoid of compressional 

 elasticity, one of the former class being simply composed of rectangular 

 parallelepipedal webs hinged together, each web consisting of three 

 systems of parallel rods freely jointed at their points of meeting. 



But we ought not to lose sight of the fact that a gyrostatic isther 

 will be effective, whatever be its modulus of compressibility, provided it 

 has no purely distortional elasticity. Thus FitzGerald's fluid need not 

 be incompressible ; an oblique parallelepipedal frame on which to mount 

 the gyrostats will do equally as well as a rectangular frame ; and we may 

 also have more complicated forms. ^ 



The wide field of physical theory which is opened up by this 

 remark that in a rotational ^ther, however heterogeneous it maybe, com- 

 pressional waves are propagated in perfect independence of rotational 

 waves, must be reserved for future consideration. A generalisation of 

 Maxwell's electrodynamic equations has been already proposed and dis- 

 cussed by von Helmholtz, which introduces the possibility of compres- 

 sional disturbances ; but that theory is on quite a difl'erent footing from 

 the one here suggested, in that Helmholtz's compressional wave interacts 

 with the rotational one, getting mixed up with it at each refraction into a 

 different medium. 



The only optical phenomena which the compression can affect, on 

 MacCuUagh's theory, appear to be magneto-optic reflexion and possibly 

 other such secondary disturbances, depending on the introduction of 

 terms of higher orders into the energy-function. 



The Bibliography of Solution. — Report of the Committee, consist- 

 ing of Professor W. A. Tilden (Chairman), Dr. W. W. J. NicOL 

 (Secretary), Professor H. McLeod, Mr. S. U. Pickering, Pro- 

 fessor W. Kamsay, and Professor Sydney Young. 



The Committee regret that but little progress has been made with their 

 work since the date of the last report. They hope, however, to complete 

 the work this year, and arrange it in a form suitable for publication. 

 They therefore desire reappointment without a grant. 



' See, however, Lord Kelvin (Sir W. Thomson), ' On the Propagation of Laminar 

 Motion through a turbulently-moving inviscid Fluid,' Phil. Mag., 1887. 



2 Lord Kelvin (Sir W. Thomson), Collected Papers, vol. iii. 1890, pp. 466-472. 



^ (Jf. J. Larmor, ' On Possible Systems of Jointed Wickerwork, and their Degrees 

 of Internal Freedom,' Proc. Camhndge Phil. Soc, 1884. 



