The electricity produced by an ordinary electric machine exhibits under certain conditions phenomena which bear a striking resemblance to the phenomena at-tendant on lightning. In both cases there is a flash of light; in both there is a report which in the case of lightning we call thunder; and in both cases intense heat is developed which is capable of setting fire to combustible bodies. Further the spark from an electric machine travels through space with extraordinary rapidity and so does a flash of lightning; the spark follows a zig-zag course and so does a flash of lightning; the spark moves silently and harmlessly through metal rods and stout wires while it forces its way with destructive effect through bad conductors and it is so too with a flash of lightning. Lastly the electricity of a machine is capable of giving a severe shock to the human body; and we know that lightning gives a shock so severe as usually to cause immediate death. For these reasons it was long conjectured by scientific men that lightning is in its nature identical with electricity; and that it differs from the electricity of our machines only in this that it exists in a more powerful and destructive form.