* 
332 On Lighining-Rods. 
mense surface of the earth, will retain no sensible portion 
it 
A lightning-rod is defined to be a conductor which the 
electric matter prefers to the surrounding bodies, in its de- 
seent to the ground for the purpose of expanding itself, and 
eommonly consists of a bar of iron elevated on the buildings 
it is intended to protect, and descends, without any divisions 
or breaks in its length, into water or moist ground. n 
the rod is not perfect in its communication with a moist soil, 
or has breaks in it, the lightning, in its course, leaves it at 
that spot, for some other near body, or divides itself between 
the two to pass more rapidly into the earth. 
‘It is proved by the experiments of MM. de Romas and 
Charles, that the higher the rod is elevated in the air, other 
circumstances being equal, the more its efficacy will be in- 
creased. It is announced that the most advantageous form 
for the extremity is that of a very sbarp cone. In this coun- 
try it is usual to have three points diverging—in Europe, on 
the continent particularly, only one is used, placed perpen- 
dicularly. ; 
How far the sphere of action of the rod extends has not 
been accurately determined ; but it is known that some 
ujldings have been struck even when they had rods attach- 
ed to them. This however has always taken place at a 
always accompanied by heat, the intensity of which depends 
upon the velocity of the current. ‘This heat is sufficient to 
make a metallic wire red hot, or to fuse or disperse it, if 
sufficiently smail, so that thin slips of copper nailed to the 
masts of vessels afford no security. The heat of ‘the elec- 
tric fluid scarcely alters the temperature of a bar of metal, 
on account of its mass ; and no instance has yet occurred of 
a bar, ofrather more than half aninch square, or of a cylinder of 
the same diameter, having been fused, or even heated red hot 
a6 gars yy of the stem, and not of the whole rod, is arg 
