522 SCIENCE PROGRESS 



Jeffery, J. B.. On the Path of a Ray of Light in the Gravitational Field of 

 the Sun, Phil. Mag. (vi), 40, 1920, 327-9. 



Ogura, K., Sur la courbure du rayon lumineux dans le champ de gravita- 

 tion, Comptes Rendus, 123, 1921, 641-3. 



Forsyth, A. R., Note on the Path of a Ray of Light in the Einstein Rela- 

 tivity Theory of Gravitational Effect, M.N., R.A.S., Ixxxii, 1921, 2-11. 



Jeffreys, H., The Effect of Gravitation on Light, Phil. Mag. (vi), 42, 1921, 

 470-1. 



Flint, H. T., On the Transformation of the Equations of Motion of the 

 Dynamics of GDntinuous Media in the Restricted Theory of Relativity, 

 ibid., 794-99. 



KoPFF, A., Bemerkung zur Rotationsbewegung im Gravitationsfeld der Sterne, 

 Phys. Zeit., xxii, 1921, 179-180, 309-10, who shows that the centrifugal 

 and Coriolis forces also appear in Einstein fields. 



A large output of work on hydrodynamics is one of the 

 phenomena of the past year. The problem of flight makes it 

 urgent to discover as much as possible about various kinds of 

 fluid motion, while much of the modern theory of meteorology 

 is based on the hydrodynamics of air. One of the classical 

 problems of hydrodynamics is that concerned with finding the 

 discontinuous stream-line motion past a barrier of more general 

 form than that of a plane wing, and the applied mathematician 

 will therefore welcome the series of volumes now being pub- 

 lished by U. Cisotti under the title " Idromecanica Plana." 

 Cisotti is himself responsible for very much of the recent work 

 on two-dimensional hydrodynamics, and in the three volumes 

 he intends to issue he will deal with the latest researches and 

 methods. The first part has just been published (Milano, 192 1). 

 It contains a very valuable chapter on the various transforma- 

 tions of the complex variables used in two-dimensional hydro- 

 dynamical problems. It seems that very little attention has 

 been paid in this country to the powerful ideas introduced by 

 Levi-Civita and his followers during the past fifteen years, and 

 that we are rather content to go on with the old methods. It 

 is true that, even with the mathematics of the French and 

 Italian writers, the direct solution of the problem of the curved 

 barrier is yet quite remote ; but Cisotti's book is nevertheless 

 welcome, because it contains the solution of many problems 

 of great interest and points the way to further useful and 

 productive research. Two papers dealing with discontinuous 

 motion are by W. B. Morton : " On the Discontinuous Flow of 

 Liquid past a Wedge," Phil. Mag. (vi), 41, 192 1, 801-8, and by 

 A. R. Richardson," Stationary Waves in Water," ibid.,^0, 97-1 10. 

 The following papers on hydrodynamical problems, including the 

 theory of the tides, should be noticed : 



RiABoucHiNSKi, D., Equations du mouvement d'un fiuide rapport^es k des 



axes mobiles, Comptes Rendus, 173, 1921, 698-701. 

 Lecornu, L., Sur le mouvement permanent des liquides, ibid., 171, 1920, 



881-5. 



