LIGHTNING-RODS. 253 



The conducting power of bodies is in the ratio of their 

 surfaces. Hence a bundle of wires, ribbons, or tubes of 

 metal, are more efficient than an equal quantity of solid, 

 round, or square rods. 



The conductors of electricity in the order of their power 

 are, copper, silver, gold, iron, tin, lead, zinc, platinum, char- 

 coal, black lead, strong acids, soot and lampblack, metallic 

 ores, metallic oxides, dilute acids, saline solutions, animal 

 fluids, sea-water, fresh water, ice, living vegetables, living 

 animals, flame, smoke, vapor and humid gases, salts, rarified 

 air, dry earth and massive minerals. 



The non-conductors in their order are, shellac, amber, 

 resins, sulphur, wax, asphalt um, glass, all vitrified bodies, 

 raw silk, bleached silk, dyed silk, wool, hair, feathers, dry 

 paper, parchment and leather, baked wood and dry vegeta- 

 bles. 



The question of the utility of lightning-rods is not clearly 

 decided ; and certainly very grave doubts exist as to the use- 

 fulness of the various complicated patent devices which are 

 hawked about the country, under the sanction of splendid 

 testimonials. 



Dr. Franklin's theory was, and he claimed to have proved 

 it by having drawn the electricity harmlessly from a cloud 

 over his kite-string, that the value of the lightning-rod 

 consists, not in its ability to receive shocks, but in the fact 

 that it taps the surcharged clouds and conveys the electricity 

 quietly to the earth. 



