520 SCIENCE PROGRESS 



when it passes through unit thickness of matter of unit density. 

 Majorana holds that the absorption or quenching of gravitation 

 is not similar to the phenomena of dielectrics or permeability, 

 but that it takes place to an extent increasing exponentially 

 with the amount of matter passed through : gravitation being 

 a flux of energy, the absorption of which causes a rise in tem- 

 perature. One interesting deduction is that the true mass of 

 the sun is about three times as great as that calculated astro- 

 nomically. In a later paper, Comptes Rendus, 173, 1921, 478-9, 

 Majorana states that a new experiment gave a value 2'$ x io~^* 

 for the quenching factor. H. N. Russell, Astro. Jour., liv, 192 1, 

 334-46, contends that these results are inconsistent with the 

 phenomena of the tides. A revival of the theory that gravi- 

 tation is a streaming in the ether is contained in a paper by 

 H. Fricke, Phys. Zeit., xxii, 192 1, 636-9. 



One of the most brilliant applications of Einstein's methods 

 is Weyl's extension of the theory to include the phenomena of 

 electro-magnetism, although the impression gathered by the 

 present writer in a conversation with Einstein was that the 

 latter is content to stop at gravitation and declines to reduce 

 other forces to properties of the space-time continuum. The 

 argument is that gravitation is unique in its universality, in 

 being a property of all matter. A further generalisation of 

 Weyl's work is made by A. S. Eddington, Proc. Roy. Soc, 99A, 

 1 92 1, 104-22. After working out a pure geometry of a more 

 generalised kind, the author shows that Einstein's mathematics 

 represent the facts of gravitation exactly. In connection with 

 the question of electro-magnetism reference should be made to 

 a very interesting paper by E. T. Whittaker, Proc. Roy. Soc. 

 Edin., xlii, 192 1, 1-23. Whittaker introduces a new kind of 

 tube of force, which he calls a " calamoid " : it is a generalisa- 

 tion of the Faraday tube, and reduces to the electric tube if there 

 is no magnetic field present, to the magnetic tube if there is no 

 electric field present. When both exist the quantity ((electric 

 force)'* — (magnetic force )'^)* is inversely proportional to the tube 

 section. 



Many investigations of a mathematical nature have been 

 suggested by relativity. An obvious problem is to discuss the 

 exact form of the path of a planet according to Einstein's theory. 

 In addition to the paper by A. R. Forsyth (see Sci. Prog., 1921, 

 526), we now have investigations in terms of elliptic functions 

 by G. Greenhill, Phil. Mag., (vi), 41, 1921, 143-8, and by F. 

 Morley, Amer. J. Math., xliii, 192 1, 29-32. W. B. Morton dis- 

 cusses the parabolic and planetary orbits, Phil. Mag. (vi), 40, 

 1920, 674-7, and ibid., 42, 1921, 511-22. Other papers are by 

 A. Anderson, ibid., 39, 1920, 626-9 ; ibid., 40, 1920, 499-501 ; 

 E. S. Pearson, ibid., 40, 1920, 342-4, who shows that Anderson 



