constitutes the dangerous ‘return stroke.’ Lord Mahon was the first to demonstrate by experiment its mode of action; as shown in the following illustration. [Illustration] A B C is the electrified cloud, the two ends of which come near the earth. The lightning discharge occurs at C. A man at F is killed by the return stroke, while those at D, nearer to the place of discharge, but further from the cloud, receive no injury. It may be mentioned that it was the action of the return shock upon the limbs of a dead frog in Galvani’s laboratory that led to the Professor’s experiments on animal electricity, and further to the discovery by Volta of that form of electrical action which bears his name. The subject of the origin of atmospheric electricity has at all times been a favourite source of speculation with scientific investigators, and given rise to numerous hypotheses. The eminent Swiss savant, Professor de Saussure, already referred to, held that all atmospheric electricity was due to the evaporation of the waters of the globe through the effect of the sun. To prove this, he made a great number of experiments, showing that whenever water, whether pure or containing more or less salt, whether acid or alkaline, is projected upon a metal crucible heated to redness, the evaporation that takes place immediately is accompanied by strong liberation of electricity. The fact is undisputed by scientific men, but not so the conclusion. Another eminent savant, no less distinguished than De Saussure, Professor De la Rive, in taking up the experiments of the former, succeeded in showing that the production of atmospheric electricity by throwing water upon heated metal was not the simple effect of evaporation, but due to chemical causes.