1908] on the Scientific Worh of Lord Kelvin. 220 



following Maxwell) that the mechanism of physical energy is so 

 minute in scale of magnitude as to be only partially under the con- 

 trol of man. Is it possible to reduce all potential energy, electric or 

 otherwise, to the ultimate simplest terms, as interactions of latent 

 cyclic motions, such as he was constantly occupying himself with, in 

 connection with his gyrostats ? 



At one period, from 1867 onward, Thomson made a very deter- 

 mined effort to carry a special scheme of this kind as far as it could 

 go, in the form of the theory of vortex atoms, to which he devoted 

 some of his most original and powerful efforts, building on the 

 famous hydrodynamic theorems of Helmholtz which have exerted so 

 great an influence in modern physical thought. To Lord Kelvin, 

 at this time,* prompted by a magnificent display of smoke rings 

 recently witnessed in Tait's lecture room where they rebounded from 

 one another as if made of rubber, the discoveries of Helmholtz in- 

 evitably suggested that vortex rings " are the only true atoms," as they 

 evade the customary " monstrous assumption of infinitely strong and 

 infinitely rigid pieces of matter," while they have permanent indi- 

 viduality and free periods of vibration, in that respect probaljly 

 passing beyond any ideas present to the mind of Rankine, when in 

 1850 he treated of ' Molecular Vortices ' in connection with thermo- 

 dynamics. The vortex ring would also be " strong and durable," an 

 unchanging element, in a way that a vibrating congeries of more 

 elementary discrete ultimate atoms could not be. 



We have also seen him engaged in the same task of illustration of 

 elasticity and other properties of matter, by systems of rigid gyrostats 

 connected by mechanism, in place of the flexible vortex rings affected 

 only by the fluid medium in which they subsist. Thus, in 1856, he 

 illustrated Faraday's magneto-optics by calculating the duplication 

 of the period of oscillation of a pendulum, which is produced when a 

 gyrostat is hung from it — a prolilem which, as Gray remarks, bears 

 directly on the mode of explanation of the modern Zeeman effect, 

 rather than of its converse the original magneto-optic influence of 

 Faraday. The general treatment of these problems of latent steady 

 motions, forming perhaps the most notable modern extension of 

 physical dynamics, originated in the second edition of Thomson and 

 Tait's ' Natural Philosophy,' published in 1879 ; the requisite analysis 

 was, however, introduced in simpler and more compact form by 

 Routh two years previously, in his essay on ' Stability of Motion,' 

 where everything was expressed in terms of a modified Lagrangian 

 function, described later l)y Helmholtz, whose writings expanded the 

 theory in many directions, and made it more widely known, as the 

 " kinetic potential." 



The recreation of yachting, by which Thomson was wont to 

 * Phil. Mag., July 1867, pp. 15-24. 



