256 MEMORIAL OF JOSEPH HENRY. 



and the second wave, being still more enfeebled, would only partially 

 de-magnetize it, leaving still a portion of the original polarity ; and 

 so for the following diminished oscillations. 



In the course of these extended researches the presence of inductive 

 action was traced to most surprising and unimagined distances. "A 

 single spark from the prime conductor of the machine, of about an 

 inch long, thrown on the end of a circuit of wire in an upper room, 

 produced an induction sufficiently powerful to magnetize needles in 

 a parallel circuit of wire placed in the cellar beneath, at a perpen- 

 dicular distance of thirty feet, with two floors and ceilings each 

 fourteen inches thick intervening." 



"The last part of the series of experiments relates to induced 

 currents from atmospheric electricity. By a very simple arrange- 

 ment, needles are strongly magnetized in the author's study, even 

 when the flash is at the distance of seven or eight miles, and when 

 the thunder is scarcely audible. On this principle he proposes a 

 simple self-registering electrometer, connected with an elevated 

 exploring rod." For obtaining the results above alluded to, a 

 thick wire was soldered to the edge of the tin roof of his dwelling 

 and passed into his study through a hole in the window frame; 

 while a similar wire passing out to the ground, terminated in con- 

 nection with a metal plate in a deep well close by. Between the 

 wire ends within his study, various apparatus, including magnetiz- 

 ing helices of different sizes *and characters, could be attached, so as 

 to be w r ithin the line of conduction from the roof to the ground. 

 The inductions from atmospheric discharges were found to have the 

 oscillatory character observed with the Leyden jar; and by inter- 

 posing several magnetizing helices with few and with many con- 

 volutions, Henry was able to get from a needle in the former 

 the polarity due to the direct current, and in the latter, that due 

 to the return current; thus catching the lightning (as it were) upon 

 the rebound. 



In examining the "lateral discharge" from a lightning-rod in 

 good connection with the earth, he had often observed that while a 

 spark could be obtained sufficiently strong to be distinctly felt, it 

 scarcely affected in the slightest degree a delicate gold-leaf electro- 

 scope. How explain so incongruous a phenomenon? Henry 



