TRANSACTIONS OF SECTION A. 625 



represent themecaanical interactions of electro-dynamic systems, as transmitted from 



Soint to point by means of simple stress, are not doomed to failure ; whether they 

 o not, in fact, introduce unnecessary and insurmountable difficulty into the theory. 

 The idea of identifying an atom with a state of strain or motion, pervading the region 

 of the EBther around its nucleus, appears to demand wider views as to what constitutes 

 dynamical transmission. The idea that any small portion of the primordial 

 medium can be isolated, by merely introducing tractions acting over its surface and 

 transmitted from the surrounding parts, is no longer appropriate or consistent : a 

 part of the dynamical disturbance in that element of the medium is on this hypo- 

 thesis already classified as belonging to, and carried along with, atoms that are 

 outside it but in its neighbourhood — and this part must not be counted twice over. 

 The law of Poynting relating to the paths of the 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 previous British familiarity with this 

 order of ideas, that the significance for general physics 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 

 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 system, 

 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 affords of the fundamental postulate of physical science, that mechanical 

 phenomena are not parts of a scheme too involved for us to explore, but rather 

 present themselves in definite and consistent correlations, which we are able to dis- 

 entangle and apprehend with continually 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 involves the idea of a mechanical connection through which that effort 

 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 fict, the basis on which Maxwell's theory had to be judtjed 

 until Hertz detected the actual working of the medium. In-^tantaae.ou^ transnis- 

 sion 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. Poincare to the International Con- 

 gress of Physics. M. Poincare accepts the principle of Least Action as a 

 reliable basis for the formulation of physical theory, but he imposes 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 he satisfied indirectly, if the effects could 

 be traced across the intervening asther 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 vorte:^ atoms, iu which the energy of the atom is distributed throughout 



' Cf. also Hertz on the electro-magnetic equations, § 13, Wt.ecl. Amu, 1800. [The 

 standpoint of Hertz's posthumous Meohanih approximates, however, to that hero 

 mamtained.] The problem of merely replacing 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. 



1900 SS 



