398 WRITINGS OF JOSEPH HENRY. [1855- 



instructive. The descent of the visible vapor in the form of 

 an inverted cone is a phenomenon which will be considered 

 of special interest, particularly by those who ascribe the 

 motive power of a tornado entirely to electricity. 



The continuance of the discharge during four seconds is 

 in accordance with other instances which have been fre- 

 quently observed, and is to be attributed to a series of dis- 

 charges in rapid succession through the same path. 



The appearance of light along the whole course of the rods 

 forming the guy may be attributed to the circumstance that 

 the metal at the time of the discharge was covered with a 

 thin stratum of water into which the electricity was pro- 

 jected by its self-repulsion, and on account of the imperfect 

 conductibility of the liquid, gave rise to the phenomena ob- 

 served. 



This may be illustrated experimentally by discharging 

 an electrical battery through a slip of tin foil wetted with a 

 thin stratum of water. The discharge which would be in- 

 sensible along the dry metal becomes luminous through its 

 whole course. 



While this account of Mr. Rogers clearly shows the at- 

 tractive power of an elevated conductor under particular 

 circumstances, it also proves the fact that an edifice may be 

 protected from harm, provided it be furnished with a suffi- 

 cient number of properly constructed rods. 



Construction of lightning-rods. — Electricity (as we have seen 

 — page 342,) tends to pass at the surface of a conductor of a 

 sufficient size, but it does not follow from this that every in- 

 crease of surface, the quantity of metal being the same, will 

 tend to diminish the resistance of the conductor to the pass- 

 age of a discharge. From an imperfect view of the subject, " 

 many persons have supposed that merely flattening the light- 

 ning-rod, and thus increasing the surface would tend to in- 

 crease the conducting power, but it must be evident from 

 the principle of repulsion, that in diminishing the distance 

 between the two flat surfaces, we tend to increase the repul- 

 sion between the atoms, which would pass parallel to the 

 axis along the middle of each flat side, and thus, though the 



