Gravitation and Light 331 



velocities beyond actual astronomical experience, not small com- 

 pared with that of light, mass comes to depend on speed; thus it 

 is not any longer available as a definite dynamical constant. On 

 the earlier uniform relativity it emerged however definitely in 

 another way as a feature of every permanent collocation of energy 

 and proportional to its amount E, equal in fact to Ejc^. This 

 follows immediately if Least Action is fundamental. Thus it is 

 grouped energy that possesses located momentum: and it is this 

 energy that has to gravitate, mass confined to matter alone having 

 proved inadequate to a Least Action formulation in the mixed 

 space-time of universal limited relativity. Dynamical principles 

 had therefore to take the form of a theory of conservation of energy 

 and of abstract momentum as they travel through a medium, at 

 the same time receiving additions by the operation of an internal 

 stress to which the medium is to be subject. In other words, 

 general dynamics cannot be more detailed than a mere description 

 of the migration of energy and of momentum in a medium under 

 the influence of some internal system of stress adjusted to fit the 

 equations as simply as possible. This stress is what has to stand 

 for or represent the agencies of nature. The theory is borrowed 

 and generalised from the Maxwellian theory of stress in the aether, 

 which was an isolated, apparently rather accidental, feature that 

 did not fit well into the substance of Maxwell's scheme, because in 

 fact it could not be connected with a strain expressive of its 

 origin. Now however, inertia of bodies having failed as the standard 

 measure of force, energy and momentum, and a postulated ad- 

 justing stress entirely at our choice, are promoted to occupy the 

 vacant place. Only it is not called a stress: the idea of a physical 

 medium is avoided, so it is named an algebraic tensor. There is 

 no law of elasticity involved, or relation of stress to strain, such 

 as makes elastic problems determinate. Thus the scheme may 

 have accidental features, is perhaps far from being unique. Another 

 parallel to it is Maxwell's theory of stresses in a gas due to varying 

 temperature: but that continuous theory could never have been 

 constructed in definite form without the foundation of the be- 

 haviour of the individual molecules. 



When however the fourfold frame is very nearly flat, the rela- 

 tions of energy-momentum-stress appear to fall in with the law of 

 gravitation, with energy as the source of its potential instead of 

 matter. 



When the deranged spacial frame nowhere differs much from 

 the flat, it may be expected that the extent of its fourfold element 

 will be altered from the value for coordinates of the corresponding 

 type on the flat only to the second order, for the same kind of 

 reason as applies in comparing a slightly deranged plane sheet with 

 the original plane. In fact, if the displacement is everywhere small, 



