74 
In my experiments on tbe 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 
currents 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, 18G8. 
