CONTENTS.
| [LECTURE I.] | [Pages 5-26] |
| [LIGHTNING AND THUNDER.] | |
| Identity of Lightning and Electricity—Franklin’s Experiment—Fatal Experiment of Richman—Immediate Cause of Lightning—Illustration from Electric Spark—What a Flash of Lightning Is—Duration of a Flash of Lightning—Experiments of Professor Rood—Wheatstone’s Experiments—Experiment with Rotating Disc—Brightness of a Flash of Lightning—Various Forms of Lightning—Forked Lightning, Sheet Lightning, Globe Lightning—St. Elmo’s Fire—Experimental Illustration—Origin of Lightning—Length of a Flash of Lightning—Physical Cause of Thunder—Rolling of Thunder—Succession of Peals—Variation of Intensity—Distance of a Flash of Lightning | |
| [LECTURE II.] | [Pages 26-53] |
| [LIGHTNING CONDUCTORS.] | |
| Destructive Effects of Lightning—Destruction of Buildings—Destruction of Ships at Sea—Destruction of Powder Magazines—Experimental Illustrations—Destruction of Life by Lightning—The Return Shock—Franklin’s Lightning Rods—Introduction of Lightning Rods into England—The Battle of Balls and Points—Functions of a Lightning Conductor—Conditions of a Lightning Conductor—Mischief Done by Bad Conductors—Evil Effects of a Bad Earth Contact—Danger from Rival Conductors—Insulation of Lightning Conductors—Personal Safety in a Thunder Storm—Practical Rules—Security Afforded by Lightning Rods | |
| [APPENDIX.] | [Pages 55-62] |
| [RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.] | |
| Theory of Lightning Conductors Challenged—Lectures of Professor Lodge—Short Account of his Views and Arguments—Effect of Self-Induction on a Lightning Rod—Experiment on the Discharge of a Leyden Jar—Outer Shell only of a Lightning Rod Acts as a Conductor—Discussion at the Meeting of the British Association, September, 1888—Statement by Mr. Preece—Lord Rayleigh and Sir William Thomson—Professor Rowland and Professor Forbes—M. de Fonvielle, Sir James Douglass, and Mr. Symons—Reply of Professor Lodge—Concluding Remarks of Professor Fitzgerald, President of the Section—Summary Showing the Present State of the Question | |
LIST OF ILLUSTRATIONS.
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.
With the magnanimity of a really great man, Franklin published this project to the world; being more solicitous to extend the domain of science by new discoveries, than to secure for himself the glory of having made them. The project was set forth in a letter to Mr. Collinson, of London, which bears date July 29, 1750, and which, in the course of a year or two, was translated into the principal languages of Europe. Two years later the experiment suggested by Franklin was made by Monsieur Dalibard, a wealthy man of science, at his villa near Marly-la-Ville, a few miles from Paris. In the middle of an elevated plain Monsieur Dalibard erected an iron rod, forty feet in length, one inch in diameter, and ending above in a sharp steel point. The iron rod rested on an insulating support, and was kept in position by means of silk cords.
In the absence of Monsieur Dalibard, who was called by business to Paris, this apparatus was watched by an old dragoon, named Coiffier; and on the afternoon of the tenth of May, 1752, he drew sparks from the lower end of the rod at the time that a thundercloud was passing over the neighborhood. Conscious of the importance that would be attached to this phenomenon, the old dragoon summoned, in all haste, the prior of Marly to come and witness it. The prior came without delay, and he was followed by some of the principal inhabitants of the village. In the presence of the little group, thus gathered together, the experiment was repeated—electric sparks were again drawn, in rapid succession, from the iron rod; the prediction of Franklin was fulfilled to the letter; and the identity of lightning and electricity was, for the first time, demonstrated to the world.