1908] on the Scientific Work of Lord Kelvin. 227 



were interpreted cumbrously in terms of the idea of description of 

 areas, introduced long before by Kepler and Newton for the simple 

 case of the motion of the planets. Building on the introduction by 

 Poinsot of the idea of rotational effort or torque, Thomson set about 

 making the rotational momentum possessed by a body an intuitive 

 fact, like its mass or volume or density, instead of a mere complex 

 mathematical expression. The subject was illustrated in his lectures 

 by experiments intended to develop a direct sense of the elastic stiff- 

 ness of balanced spinning masses. The gyroscope of Foucault, with 

 its special gimbal suspension, became the gyrostat, in which the spin- 

 ning body was enclosed and protected by a frame or box, and could 

 thus be manipulated readily in many ways, e.g., either balancing 

 itself on an edge, or on stilts, or being hung on to a pendulum with 

 free suspension ; it could thus assist towards realising varied know- 

 ledge of the properties conferred by this newly recognised type of 

 permanent possession of matter — the intrinsic spin of a free body, 

 which in the absence of friction of the bearings would be retained 

 for ever — which has in fact more recently become a branch of experi- 

 mental engineering. 



This idea of spin, as a possible essential endowment of masses of 

 matter, was constantly being extended in many directions. In par- 

 ticular it came to be the foundation of the first, and hitherto the 

 only fairly successful, mechanical representation of the working of 

 the aether of space. Though Thomson's early mathematical elucida- 

 tions and demonstrations of Faraday's ideas formed a main part of 

 the material which Maxwell welded into a connected theory of elec- 

 tricity, which afterwards extended itself into optics and general 

 radiation, his own mode of investigation took on rather the reverse 

 order. For he could imagine no direct mechanical model wide 



enouo-h to include the various sensible modes of workino^ of elec- 



. . . . 



tricity, which he had himself elucidated in special domains from the 



doctrine of transformation of energy and the principles of Ampere 

 and Faraday. Thus he turned aside to attack the more compact and 

 definite problem of discovering the type of an aether that would 

 satisfy merely vibratory requirements, those namely of light and 

 radiation. Long-continued efforts to base its properties on elasticity 

 arising simply from solidity, after the manner developed by Green, 

 though evolving many fruitful aspects of elastic propagation in con- 

 tinuous physical media, had all ended in failure. Why not then try 

 this elasticity of rotation instead of elasticity of form ? The result 

 was immediate success. But, as embodied in a model — only theoretical 

 it is true, but one whose construction could be distinctly contemplated, 

 and has since been in part made practically effective in the form of 

 gyrostatic apparatus for steadying ships relatively to the vertical — the 

 achievement carried wider consequences than this. It formed the 

 objective justification of a purely analytical solution of the same 

 problem which had been offered by MacCullagh, of Dublin, fifty 



Q 2 



