﻿.Einstein and Grossmann's Theory of Gravitation. 11 

 by the equation 



i-^ly/i (11)his 



4. Collapse rings can be advantageously employed when, 

 and only when, the dimensions of a tube are such that 





■..-.. (18) bit 



In the equations (11), (12), (17) and (18), t denotes the 

 thickness and d the diameter of the tube : the3e quantities, 

 and s, must be expressed in terms of the same units ; y c is 

 the stress at the yield-point in compression (for practical 

 purposes the yield-point in tension may be substituted), and 

 E is Young's modulus for the material of the tube : these 

 quantities, and :p, must be expressed in terms of the same 

 units. 



For additional security, it would perhaps be advisable 

 to substitute 1*5 for the factor 1"73, equation (17) having 

 been based on an estimate (15) for the collapsing pressure 

 of long tubes which is somewhat in excess of Carman's. 

 Moreover, the figure 1*73 is based solely upon equation (1), 

 and, as Mr. Cook has remarked *, his tests cannot be 

 regarded as sufficient in number or covering a great enough 

 range of dimensions to confirm this equation definitely. 



August 21, 1914. 



X. A Summary of Einstein and Grossmann s Theory of 

 Gravitation. By Dr. A. D. Fokker {Leiden) f. 



1. T)EHHAPS it might be useful to give a brief account 

 JL of the principal features of Einstein and Grossmann's 

 gravitation theory J, leaving aside as far as possible the 

 mathematical complications, but emphasizing the simple and 

 fundamental physical points. 



Unfortunately, this theory will be of little direct importance 

 to experimental physics. Not because it fails to indicate 

 any experiments which could bring evidence for its validity 

 or non-validity, but because the foreseen effects probably are 

 far too small to be detected by present experimental methods. 



* Loc. cit. p. 56. 



t Communicated b}' Prof. W. II. Bragg, F.R.S. 



| WV(JJJ.ljLlU.jUH^n.tCU. U> X 1U1, II . J.J.. J_>1 tl^ii , J. .XX.KJ. 



t Entwurf einev Verallgemeinerten HeUttivitatstheorie und einer Theorie 



der Gravitation, Phys. Teil v. A. Einstein, Math. Teil v. M. Grossmann, 

 1913 (Teubner). A. Einstein, Phys. Zeitschr. xiv. p, 125] (1918), 



