464 SCIENCE PROGRESS 



ised Relativity Theory." The inclusion of gravitation within 

 any of the recognised electric theories has always presented 

 considerable difficulties, based in all probability on the fact 

 that the known laws of gravitational action are so exceedingly 

 simple, paradoxical as that statement may appear. As the 

 subject left Newton's hands, no question was raised as to the 

 possibility of the gravitational action of one body on another 

 being an influence which requires time for its propagation from 

 the one body to the other. With the advance of elastic solid 

 and electromagnetic theories of the ether, such views as to the 

 propagation of gravitation through space with a finite velocity 

 naturally came forward, and when Einstein's relativity theory 

 began to be accepted with its necessary restriction that nothing 

 could travel faster than light, the question was seriously raised 

 as to the possibility of reconciling astronomical phenomena 

 with the view that gravitational action was propagated through 

 space with the velocity of light, a view which would accord 

 with the theory that gravitation is the result of an " uncom- 

 pensated residue of elctrical forces " (to quote Prof. Richard- 

 son). Now Lorentz has considered this question very fully, 

 and finds that if gravitational action is propagated in the same 

 manner as electrical actions, it will give rise to effects practically 

 identical with those which follow from the usual Newtonian 

 law. The differences are too small to be detected, and they are 

 incapable of accounting for certain recognised discrepancies 

 in the motions of the heavenly bodies. Relativity theory would 

 therefore treat Newton's theory as a first approximation, but 

 without any new experimental indications as to the direction 

 in which an extension is to be sought ; as mentioned, the sim- 

 plicity of the known gravitational laws permit, as it were, too 

 much freedom ; there is but little restriction on the number of 

 possible assumptions and insufficient definition of the direction 

 in which progress is to be made. 



In his " Generalised Relativity Theory" Einstein makes an 

 attempt to develop gravitational theory in a manner consistent 

 with known facts, so as to bring it within the scope of electro- 

 magnetic actions ; in doing so he has scored a notable success 

 in removing one of the most serious discrepancies known between 

 astronomical theory and fact. In the earlier relativity theory 

 Einstein assumed the equivalence of two systems in uniform 

 relative motion as regards the summarising of phenomena in 



