74 



In my experiments on the electrical condition of the terres- 

 trial globe* I have already directed attention to the powerful 

 influence which lines of metal, extended in contact with 

 moist ground, exercise in promoting the discharge of electric 

 cuiTents of comparatively low tension into the earth's 

 substance, and also that the amount of the discharge from 

 an electro-motor into the earth increases conjointly with the 

 tension of the current and the length of the conductor 

 extended in contact with the earth. It is not, therefore, 

 surprising that atmospheric electricity, of a tension sufficient 

 to strike through a stratum of air several hundred yards 

 thick, should find an easier path to the earth by leaping 

 from a lightning conductor through a few feet of air or stone 

 to a great system of gas and water mains, extending in large 

 towns for miles, than by the short line of metal extended in 

 the ground which forms the usual termination of a lightning 

 conductor. 



It deserves to be noticed that in the cases of lightning 

 discharge which I have cited, the lightning conductors 

 acted efficiently in protecting the buildings from damage 

 of a mechanical nature — the trifling injury to the church 

 tower at Kersal Moor being directly attributable to the 

 presence of the gas pipe in proximity to the conductor. 

 Nor would there have been any danger from fire by the 

 ignition of the gas if all the pipes used in the interior of 

 the buildings had been made of iron or brass instead of lead . 

 for all the cases of the ignition of gas by lightning, which 

 have come under my observation, have been brought about 

 by the fusion of lead pipes in the line of discharge. The 

 substitution of brass and iron, wherever lead is used in the 

 * Philosophical Magazine, August, 1868. 



