LETTER OF J. HENRY TO REV. S. B. DOD. 153 



SO distant that the thunder could scarcely be heard. The electrical 

 disturbance produced in this case was also found to be of an oscil- 

 latory 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 number of turns ; for example, in a spiral of a 

 single turn the needle would be magnetized j^Ztts, or in the direction 

 due to the first and more powerful wave. By increasing the num- 

 ber 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 further increasing the 

 number of turns, the third wave would be so exalted as to neu- 

 tralize the effects of the preceding two, and so on. In the case of 

 induction by lightning, the same result was obtained by placing a 

 number of magnetizing spirals, of different magnetizing intensities, 

 in the opening of the primary conductor, the result of which was 

 to produce 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 scries of 

 experiments 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 

 fire to delicate inflammable substances. The spark thus given off 

 was found to be of a peculiar character, for while it produced com- 

 bustion and gave a slight shock, and fired the electrical pistol, it 

 scarcely at all affected a gold leaf electroscope. Indeed, it consisted 

 of two sparks, one from the conductor and the other to it, in such 

 (piick succession that the rupture 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 point in succession 

 received a charge analogous to the statical charge of a prime con- 

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



