162 BIOGEAPHICAL MEMOIR OF JOSEPH HENRY. 



of the wliole being coiled around an insulated drum. I was assisted in 

 these experiments by Mr. Brown, of Philadelphia, who furnished the 

 kites. When they were elevated, at a time when the sky was peifectly 

 clear, sparks were drawn of surprising intensity and pungency, the elec- 

 tricity being supplied from the air, and the intensity being attributed to 

 the induction of the long wire on itself. 



VI. The next series of exi^eriments pertaining to the same class was 

 on the induction from thunder clouds. For this purpose the tin cover- 

 ing of the roof of the house in which I resided was used as an inductive 

 l)late. A wire was soldered to the edge of the roof near the gub'^er, was 

 passed into my study and out again through holes in the window-sash, 

 and terminated in connection with a i)late of metal in a deep well im- 

 mediately in front of the house. By breaking the continuity of that 

 part of the wire which was in the study, and introducing into the open- 

 ing a magnetizing spiral, needles placed in this could be magnetized by 

 a flash of lightning so distant that the thunder could scarcely be heard. 

 The electrical disturbance produced in this case was also found to be of 

 an oscillatory character, a discharge first passing through the wire from 

 the roof to the well, then another in the opposite direction, and so on 

 until equilibrium was restored. This result was arrived at in this case, 

 as well as in that of the Leyden jar, before mentioned, by placing the 

 same, or a similar needle, in succession, in spirals of greater and greater 

 numbers of turns ; for example, in a spiral of a single turn the needle 

 would be magnetized plus, or in the direction due to the first and more 

 XDOwerful wave. By increasing the number of coils, the action of the 

 second wave became dominant, so that it would more than neutralize 

 the magnetism produced by the first wave, and leave the needle minus. 

 By farther increasing the number of turns, the third wave would be so 

 exalted as to neutralize the effects of the preceding two, and so on. In 

 the case of induction by lightning, the same result was obtained by plac- 

 ing a number of magnetizing spirals, of different magnetizing intensities, 

 in the opening of the primary conductor, the result of which was to pro- 

 duce the magnetization of an equal number of needles, plus and minus, 

 indicating alternate currents in opposite directions. 



VII. In connection with this class of investigations a series of experi- 

 ments was made in regard to lightning-rods. It was found that when a 

 quantity of electricity was thrown upon a rod, the lower end of which 

 was connected with a plate of metal sunk in the water of a deep well, 

 that the electricity did not descend silently into water, but that sparks 

 could be drawn from every part of the rod sufficiently intense to explode 

 an electrical pistol, and to set fii-e to delicate inflammable substances. 

 The spark thus given oft" was found to be of a peculiar character, for 

 while it produced combustion and gave a slight shock, and fired the 

 electrical pistol, it scarcely at all affected a gold-leaf electroscope. In- 

 deed, it consisted of two sparks, one from the conductor and the other 

 to it, in such quick succession, that the mixture of the air by the first 

 served for the path of the second. The conclusion arrived at was, that 

 during the passage of the electricity down the rod, each j)oint in succes- 

 sion i^eceived a cliarge analogous to the statical charge of a i^rime con- 

 ductor, and that this charge, in its passage down the rod, was immedi- 

 ately preceded by a negative charge ; the two in their passage past the 

 point at which the spark was drawn, giving rise to its duplex character. 

 It was also shown by a series of experiments in transmitting a powerful 

 discharge through a portion of air, that the latter, along the path of 

 discharge, was endowed for a moment with an intense repulsive energy. 

 So great is this that in one instance, when an electrical discharge from 



