Introduction; Part I. The Unconnected Manifold: 1. Invariance; 2. Integrals; Part II. Affinely Connected Manifold: 3. Invariant derivatives; 4. Some relations between ordinary and invariant derivatives; 5. The notion of parallel transfer; 6. The curvature tensor; 7. The geodesics of an affine connexion; 8. The general geometrical hypothesis about gravitation; Part III. Metrically Connected Manifold: 9. Metrical affinities; 10. The meaning of the metric according to the special theory of relativity; 11. Conservation laws and variational principles; 12. Generalizations of Einstein's theory.