290 WRITINGS OF JOSEPH HENRY. [1850 



ON THE EXPANSIVE ENERGY OF LIGHTNING STROKES.* 



(Proceedings American Association, Adv. of Science, vol. iv, pp. 7, 10.) 



JAtignst 19, 1850. 



Professor Henry mentioned an instance of an explosion 

 during the passage of an electrical discharge through a 

 house, from which fact he had been led to the same conclu- 

 sions on this point with Professor Olmsted. He had him- 

 self made a series of experiments to ascertain whether the 

 hypothesis is true or not. The results were attained by 

 means of Kinnersley's air thermometer. His investigations 

 convinced him that the effect is due to a sudden repulsive 

 energy imparted to the air. He cited several instances — 

 some of which were noticed by himself at Princeton, where 

 the roof of one house was blown off, and the side of another 

 blown out. He considered that the great mechanical effects 

 of an electrical discharge are due in most cases to an expan- 

 sive or repulsive power in the air. He had made some in- 

 teresting experiments in galvanism, the effects of which he 

 referred to the same cause. - - - 



Professor Henry mentioned instances where ordinary elec- 

 trical discharges had affected a circle of twenty miles in 

 diameter. By means of an apparatus simply constructed 

 for the occasion, he had succeeded in magnetizing a needle 

 by a flash of lightning so far off that he could not hear the 

 thunder. He explained the apparatus. He considered that 

 every flash of such electricity produces effects to great dis- 

 tances, and may perhaps affect half the globe. 



*[Kemarks on a communication, by Professor Dennison Olmsted, of Yale 

 College, to the Association, on "Notes of some points of Electrical Theory." 

 Professor Olmsted illustrated by several observations the expansive energy 

 of lightning strokes, and also the occurrence of the lateral discharge from 

 good conductors well connected with the earth.] 



