432 



SCIENCE. 



[N. S. Vol. XII. No. 299. 



transmission of energy is known to hold in 

 its simple form only when the electric 

 charges or currents are in a steady state ; 

 when they are changing their positions or 

 configurations their own fields of intrinsic 

 energy are carried along with them. 



It is not surprising, considering the pre- 

 vious British familiarity with this order of 

 ideas, that the significance for general phys- 

 ics of Helmholtz's doctrine of vortices was 

 eagerly developed in this country, in the 

 form in which it became embodied through 

 Lord Kelvin's famous illustration of the 

 constitution of the matter, as consisting of 

 atoms with separate existence and mutual 

 interactions. This vortex atom theory has 

 been a main source of physical suggestion 

 because it presents, on a simple basis, a 

 dynamical picture of an ideal material sys- 

 tem, atomically constituted, which could go 

 on automatically without extraneous sup- 

 port. The value of such a picture may 

 be held to lie, not in any supposition that 

 this is the mechanism of the actual world 

 laid bare, but in the vivid illustration it 

 afi"ords of the fundamental postulate of 

 physical science, that mechanical phenom- 

 ena are not parts of a scheme too involved 

 for us to explore, but rather present them- 

 selves in definite and consistent correlations, 

 which we are able to disentangle and appre- 

 hend with continuously increasing precision . 

 It would be an interesting question to 

 trace the origin of our preference for a 

 theory of transmission of physical action 

 over one of direct action at a distance. It 

 may be held that it rests on the same order 

 of ideas as supplies our conception of force ; 

 that the notion of effort which we associate 

 with change of the motion of a body in- 

 volves the idea of a mechanical connection 

 through which that efi"ort is applied. The 

 mere idea of a transmitting medium would 

 then be no more an ultimate foundation for 

 physical explanation than that of force 

 itself. Our choice between direct distance 



action and mediate transmission would 

 thus be dictated by the relative simplicity 

 and coherence of the accounts they give of 

 the phenomena : this is, in fact, the basis 

 on which Maxwell's theory had to be judged 

 until Hertz detected the actual working of 

 the medium. Instantaneous transmission 

 is to all intents action at a distance, except 

 in so far as the law of action may be more 

 easily formulated in terms of the medium 

 than in a direct geometrical statement. 



In connection with these questions it may 

 be permitted to refer to the eloquent and 

 weighty address recently delivered by M. 

 Poincar6 to the International Congress of 

 Physics. M. Poincar6 accepts the principle 

 of Least Action as a reliable basis for the 

 formulation of physical theory, but he im- 

 poses the condition that the results must 

 satisfy the Newtonian law of equality of 

 action and reaction between each pair of 

 bodies concerned, considered by themselves; 

 this, however, he would allow to be satisfied 

 indirectly, if the effects could be traced 

 across the intervening aether by stress, so 

 that the tractions on the two sides of each 

 ideal interface are equal and opposite.* As 

 above argued, this view appears to exclude 

 ah initio all atomic theories of the general 

 type of vortex atoms, in which the energy 

 of the atom is distributed throughout the 

 medium instead of being concentrated in a 

 nucleus ; and this remark seems to go to 

 the root of the question. On the other 

 hand, the position here asserted is that re- 

 cent dynamical developments have permit- 

 ted the extension of the principle of Action 

 to systems involving permanent motions, 

 whether obvious or latent, as part of their 

 constitution ; that on this wider basis the 



* Cf. also Hertz on the electro- magnetic equations, 

 § 12, Wied. Ann., 1890. The problem of merely re- 

 placing a system of forces by a statical stress is widely 

 indeterminate, and therefore by itself unreal ; the 

 actual question is whether any such representation 

 can be coordinated with existing dynamics. 



