WITH regard to Prof. P. A. M. Dirac's recent letter to NATURE1, I have shown, in various contexts2 that the relation between, the constant of gravitation, and t, the epoch, is given by where M0 is the apparent mass of the fictitious homogeneous universe. (The actual mass must be infinite.) With t = 2 × 109 years, this gave M0 = 2.4 × 1055 grams = mass of 1.5 × 1079 protons. Two points of interest (amongst others) emerge from the treatments I have given. First, (1) is a purely macroscopic formula, having no connexion with atomicity. It is derived from purely kinematic considerations, involving no appeal to any empirical dynamical laws, still less to atomic laws. In papers already communicated for publication, I have extended the application of (1) to all local gravitational situations and derived the inverse square law of gravitation in relativistic form in flat space, again without recourse to empirical appeals, by kinematic methods.