46 BOARD OF AGRICULTURE. 



termine the largest diameter or thickness of metal, that has ever 

 been melted by it. The loose expression of ancient writers, that 

 "the sword is liquified in the scabbard,"' is not sufficiently 

 definite for modern science. What are the actual dimensions of 

 metals actually fused by lightning? From nearly a score of 

 examples, 1 find that a copper i*od two-tenths of an inch in 

 diameter has been thus melted ; that Franklin in 1787, found that 

 a stroke of lightning had melted, at his own house in Philadelphia, 

 a conical copper rod 9| inches long, and rather more then three- 

 tenths of an inch thick at its base ; that " a thunderstroke may 

 fuse completely and throughout its extent an iron chain of 130 

 feet in length, the diameter of the iron forming the links being .23 

 of an inch, and a conical iron rod .43 of an inch thick at its base — 

 that an iron rod .47 of an inch in diameter was broken by a heavy 

 stroke of lightning, but showed no trace whatever of fusion. 

 From such data a tolerably correct idea can be formed of the 

 necessary magnitude of metallic rods to convey away without 

 harm any probable charge of the electric fluid. A stroke not 

 sufficiently powerful to fuse small rods or wires may have the 

 effect of shortening them. A metallic wire 16| feet in length has 

 been thus contracted between two and three inches. Wires 

 stretched between fixed points are thus often broken by lightning 

 strokes. (Is it not possible that the contraction results from a 

 lateral escape of the electric fluid, giving rise to tension in that 

 direction, just as a rope is shortened when from any cause its 

 diameter is increased ?) 



Lightning frequently fuses and instantly vitrifies certain earthy 

 substances. Lightning tubes or fulgurites (as they are termed) 

 are produced when lightning descends into sandy soils, the path 

 of the lightning being marked by a tube of vitrified sand. These 

 tubes are sometimes three inches in external diameter, and have 

 been known to exceed thirty-three feet in length. They generally 

 descend vertically into the sand, but are often found inclined to 

 the horizon at an angle of 40 degrees. The} r contract in descend- 

 ing and often terminate iu a point ; sometimes, however, the 

 principal tube divides into two or three branches, each with 

 smaller lateral branches from an inch to a foot in length. These 

 branches are conical and all terminate in points. The sides of the 

 tubes vary from two-hundredths of an inch to an inch in thickness. 

 The inside part of lightning tubes is smooth and bright. It 

 scratches glass and strikes fire as a flint. 



