Last summer session, with the assistance of Messrs Salvesen, Connor, and Stewart, I applied the method of measuring great differences of potential, described in a paper by Mr Paton and myself (Proc. Feb. 19, 1877), to investigate the laws of passage of the electric spark. The method essentially consists in connecting the prime conductor of the Holtz machine, not with the electrometer directly, but with an insulated spherical ball. This ball acts inductively upon another insulated spherical ball which is in connection with the electrometer.