340 WRITINGS OP JOSEPH HENRY [1855- 



sufficient quantity of the fluid could accumulate to force its 

 passage through the obstacle. An illustration of this action 

 is presented in the fact, that at the point where the lightning 

 leaves a conductor, and also where it is received by another 

 conductor, signs of fusion or of more intense action are al- 

 ways exhibited. An effect of lightning described by Pro- 

 fessor Olmsted, at a meeting of the American Association, 

 in New Haven, may be explained on this principle. A row 

 of five or six milk-pans, placed in the open air on a bench, 

 was struck by a discharge from a cloud. The electricity 

 passed through the whole series, making two holes in 

 each pan, at opposite extremities of the diameter, or at the 

 places where the electricity may be supposed to have en- 

 tered and gone out. 



There is another circumstance connected with the dis- 

 cliiarge of electricity — having an important bearing on the 

 construction of lightning-rods, which may be mentioned in 

 this place. When the repulsion of the atoms of electricity 

 in a conductor or in a cloud and the attraction of the un- 

 saturated matter below become so intense as to cause a rup- 

 ture in the air, the electricity of the cloud is j^recipitated 

 upon the conductor, and not only restores the natural quan- 

 tity, but also gives it for a moment a redundancy of elec- 

 tricity, a fact which must be evident from the theory, when 

 we consider the distance at which the induction is com- 

 municated. As this charge of free electricity passes down 

 the rod to the earth, for example, it assumes the charac- 

 ter of a wave, rendering the metal negative in advance ; 

 and thus in the transmission of free electricity through a 

 rod of metal, the action consists of two waves, one of re- 

 dundancy, immediately preceded by one of deficiency. 

 Hence if a small ball connected with the earth by a wire be 

 brought near a conductor (for example a lightning-rod) on 

 the upper end of which, discharges of electricity are thrown 

 from an electrical machine, sparks may be drawn from the 

 rod, however intimately it may be connected with the earth 

 below. 



This effect was strikingly exhibited by an experiment 



