1 IN THE MONOGRAPH SERIES directed by Henri Villat, several fasci- cules have been devoted to Relativity. First there are the general presentations ofTh. De Donder (nos. 8, 14, 43, 58), and then those more specifically devoted to Einsteinian gravitation - notably Georges Darmois's contribution (no. 25) and that of J. Haag (no. 46) on the Schwarzschild problem. The present fascicule takes its place alongside the two latter monographs, but it has been conceived and composed in such a way that it may be read and understood by anyone with a knowledge of the principles of Absolute Differential Calculus and of Relativity - either from the original exposi- tions of Einstein, Weyl, or Eddington, or, in French, from Cartan's excel- 2 lent works (for everything having to do with mathematical theories) and 3 from Chazy's (for Relativity and Celestial Mechanics), or naturally from Levi-Civita's The Absolute Differential Calculus (first edition, London and Glasgow, Blackie and Son, 1927) where the two original papers written in Italian are brought together: namely, Calcolo differenziale assoluto and Fondamenti di meccanica relativistica (Bologna, Zanichelli). As for the present fascicule, it is hardly necessary to point out that, as its title indicates, we seek to establish in the simplest possible terms the rela- tivistic aspect of what Newton and those who followed him regarded as the key to ordinary Celestial Mechanics.