THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE, 17 ORIGIN OF SUCCESSIVE PEALS OF THUNDER, 22 VARIATIONS OF INTENSITY IN A PEAL OF THUNDER, 24 DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES, 27 GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY, 32 GUN COTTON SET ON FIRE BY ELECTRIC SPARK, 33 VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK, 34 THE RETURN SHOCK ILLUSTRATED, 35 PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR, 48 INDUCTION EFFECT OF LEYDEN JAR DISCHARGE, 56 LECTURE I. LIGHTNING AND THUNDER. The electricity produced by an ordinary electric machine exhibits, under certain conditions, phenomena which bear a striking resemblance to the phenomena attendant 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. =Identity of Lightning and Electricity.=--But it was reserved for the celebrated Benjamin Franklin to demonstrate the truth of this conjecture by direct experiment. He first conceived the idea of drawing electricity from a thundercloud in the same way as it is drawn from the conductor of an electric machine. For this purpose he proposed to place a kind of sentry-box on the summit of a lofty tower, and to erect, on the sentry-box, a metal rod, projecting twenty or thirty feet upward into the air, pointed at the end, and having no electrical communication with the earth. He predicted that when a thundercloud would pass over the tower, the metal rod would become charged with electricity, and that an observer, stationed in the sentry-box, might draw from it, at pleasure, a succession of electric sparks.