326 Sir Joseph Larmor 



could not permit this transformation: nor could the fourfold 

 uniform continuum of interlaced space and time of the earlier 

 relativity theory be adapted to it. Will such a fourfold, deformed 

 into a non-uniform and therefore non-flat heterogeneous space, 

 permit it? This is the problem raised by Einstein's idea of the 

 relativity of gravitational force. Perhaps it goes even further, and 

 asks whether if this will not do, there can be some other corpus 

 of abstract differential relations invented, that will transcend 

 the notion of spacial continuity altogether but will in compen- 

 sation for that formidable complexity succeed in effecting this 

 object. 



In any case we may recognise that this merging of all the forces 

 of nature into spacial relations satisfies one requirement which is 

 not quite the claim that is explicitly made for it. The question 

 is immediately insistent; why should intrinsic forces be measurable 

 with Newton in terms of second gradients of type (Ps/dt^ and not 

 by a more complex formula involving others as well? The answer 

 supplied by the theory would be that the idea of the curvature of 

 a deranged space is expressed by a measure which does not involve 

 higher gradients. 



It is interesting to reflect nowadays that in referring to the 

 doctrines of action at a distance in the preface to the Electricity 

 and Magnetism, in 1873 Maxwell classifies them as "the method 

 which I have called the German one," and that notwithstanding 

 Helmholtz's very powerful critical work on Maxwell's theory, be- 

 ginning in 1870, that description remained substantially true until 

 after Maxwell's death in 1879. Though he lived for nine years 

 longer he seems to have taken no part in these discussions with 

 exception of a reference to Helmholtz in connexion with Weber's 

 theory {Treatise, § 254), but worked chiefly at the development of 

 the theory of stresses in gases regarded as molecular media, and 

 so in some respects parallel to his theory of an electric medium. 

 He seems to have been content to leave his electric scheme to 

 germinate and expand in the fulness of time. In connexion with 

 the recent efliorts to transcend both action at a distance and an 

 aethereal medium, his explanations, in an Appendix to the Memoir 

 on the determination of the ratio of the electric units, Phil. Trans. 

 1868 and the critical chapter on ' Theories of Action at a Distance' 

 in the Treatise, §§ 846 — 866, are far from being obsolete. 



This hypothesis as to gravitation, which asserts that it is 

 essentially of the same nature as the apparent increase of weight 

 which is experienced by an observer going up in a lift with ac- 

 celerated motion, naturally involves many consequences, and 

 raises questions regarding the relation of gravitation to physical 

 agencies such as light, the answer to which may be ambiguous until 

 yet further postulates intervene. 



