LONDON. Royal Society, Feb. 17.—G. C. Simpson: The mechanism of a thunderstorm. The theory that the separation of electricity is brought about by the breaking of raindrops is adopted. The orders of magnitude of the meteorological and electrical quantities involved are in accordance with observations. The observations made by Schonland and Craib in South Africa of changes of electrical field strength produced by lightning discharges are in complete accord with the theory.