248 PHYSICS. 
be perfectly conducting, no interruption of continuity taking place between 
the point and the ground; and all parts of the apparatus must be of the 
proper dimensions. The great advantage of a point consists, according to 
Pouillet, in this: that when a thunder cloud passes over the rod and decom- 
poses its combined electricity by induction, by repelling the like and 
attracting the like kind, the latter can stream out into the air from the point. 
In this way no accumulation of electricity in the rod can take place, and no 
danger will be experienced in approaching it, or even from coming into 
actual contact. 
Pl. 27, figs. 14—19, represent a lightning rod as recommended by Gay 
Lussac. This rod is about twenty-seven or twenty-eight feet long, and 
consists of three pieces, an iron rod 25? feet long, a brass piece of two feet, 
and a platinum needle of from one to two inches in Jength, together forming 
a cone tapering gradually above ( fig. 15). The brass rod is screwed into the 
iron rod and secured by pins. The platinum needle is soldered to the brass 
rod by silver solder, and the joint surrounded by a copper nut, as in fig. 16. 
The iron rod consists sometimes, for more easy transportation, of two ‘or 
more pieces screwed into one another and fastened by pins. Fig. 14 
represents three different modes of attaching the rod to a building. Under 
the rod, about two or three inches from the roof, a plate, bb’ ( fig. 17), 
is screwed to carry off the water; one or two inches above this plate the 
rod must be cylindrical and well turned, in order that a hinged ring, ZI’ 
( figs. 17 and 18), may be applied, to which the conducting rod can be 
attached. This latter is a quadrangular iron rod of seven to nine lines in 
thickness, screwed to the ring //', and carried over the roof, and along the 
house to the ground. Here it should terminate in several branches and 
windings, dipping into a constant current of water, or into a hole bored to a 
depth at which water exists, and filled up with powdered charcoal. Should 
there be no water in any way accessible, the rod should at least be carried 
through a channel filled with charcoal to a damp place. Instead of the 
conducting rod we may use a rope of twisted copper wire (fig. 19). A well- 
constructed rod of the dimensions just given, will protect a space of about 
sixty feet radius ; and generally, a projecting rod will protect an area whose 
radius is twice the height of the rod. Should the rod project from the roof 
of a house, we must estimate the amount of protection extended to the 
house by the elevation of the rod above the roof. 
The straw-rope hail and lightning conductor of La Postolle, pronounced 
perfectly useless on its first announcement, by the French Academy, has 
been already mentioned under the subject of Hail. 
The inefficiency of all the earlier protective means, as the burning of 
great fires, the firing of cannon, &c., is now universally recognised. The 
ringing of bells, customary during thunder storms in olden times, and still 
practised in the Tyrol, is not only useless but dangerous to the persons 
concerned, who thus complete the electric communication between the bell, 
through the rope, and the earth. It has been estimated that in the short 
space of thirty-three years, not less than 386 church steeples have been 
struck, and 121 persons engaged in ringing the bells killed. 
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