TABLE OF CONTENTS. Page. 9 16 22 Franklin's kite experiment.... The electrification of the atmosphere Some measurements of the potential of the air Experiments at the Smithsonian Institution ... Experiments at the top of the Washington Monument. Increase of potential with elevation .... The electricity of the upper air as manifested in auroral displays ILLUSTRATIONS. Page. I.-Average number of thunderstorm days in the United States..... II.-Effect of lightning stroke upon a walnut tree, January, 1899..... III.-Effect of lightning stroke upon a red oak, summer of 1898... ACKNOWLEDGMENTS. In the following pages generous use has been made of articles published by the author in The Century Magazine, Harper's Magazine, Appleton's Popular Science Monthly, Terrestrial Magnetism, and Bulletins of the United States Weather Bureau, and a grateful acknowledgment is here made of my indebtedness to these sources. I must also acknowledge the kindness of my friend Mr. Alexander J. Wurtz, of the Westinghouse Company, who cheerfully put at my disposal his notes covering many years' experiments upon the protection of electrical apparatus from lightning. 8 ALEXANDER G. MCADIE. PART I. LIGHTNING AND THE ELECTRICITY OF THE AIR. BY ALEXANDER G. McADIE. FRANKLIN'S KITE EXPERIMENT. One hundred and sixty years ago a ragged colonial regiment drew up before the home of its philosopher-colonel and fired an ill-timed salute in his honor. A fragile electrical instrument was shaken from a shelf and shattered. Franklin doubtless appreciated the salute and regretted the accident. In the course of his long life he received other salutes, as when the French Academy rose at his entrance, and he constructed and worked with other electrometers; but for us that first experience will always possess a peculiar interest. The kite and the electrometer betray the intention of the colonial scientist to explore the free air, and, reaching out from earth, study air electrification in situ. He made the beginning by identifying the lightning flash with the electricity developed by the frictional machine of that time. A hundred patient philosophers have carried on the work, improving methods and apparatus, until to-day we stand upon the threshold. of a great electrical survey of the atmosphere. It is no idle prophecy to say that the twentieth century will witness wonderful achievements in measuring the potential of the lightning flash, in demonstrating the nature of the aurora, and in utilizing the electrical energy of the cloud. The improved kite and air-runner will be the agency through which these results will be accomplished. The famous kite experiment is described by Franklin in a letter dated October 19, 1752: FRANKLIN'S ELECTRICAL MACHINE. Make a small cross of light sticks of cedar, the arms so long as to reach to the four corners of a large, thin silk handkerchief when extended. Tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite which, being properly accomodated with a tail, loop, and string, will rise |