28 Professor Leslie o?i Electrical Theories . 
sort of protection. Nay, we shall be convinced, that fully an 
equal proportion of the buildings armed with such supposed 
safeguards, have been struck with lightning. But if thunder- 
rods are useless, they are also innocent ; and, that they pro- 
voke the shaft of Heaven, is the suggestion of superstition rather 
than of science. The cloud exerts an attraction, indeed, upon 
the surface of the ground, but the force depends solely on the 
distance, and is not, in the least degree, affected by the shape 
or quality of the substances below. It rolls towards the nearest 
and most elevated objects, and strikes indiscriminately a rock, 
a tree, or a spire. 
If a thunder-rod be then an harmless, though idle appendage 
of a house, why awaken uneasy apprehensions ? It might at 
least inspire confidence in the moments of danger ; and if hap- 
piness consists merely in idea, why not indulge delicious error ? — - 
Yet, though the inevitable stroke cannot be turned aside, its 
destructive effects may be lessened; and an investigation of 
the real action of thunder will conduct us to the proper princi- 
ples. 
We have already seen, that electricity is never communicated 
from one body to another, but by the intervention of some me- 
dium. If this be extremely rare, and also a very slow conduc- 
tor, such as air, the particles contiguous to the electric body 
may be projected from it, and those, being again succeeded by 
others, may thus gradually form the communication. But if 
the medium is either a solid, or a fluid of considerable density, 
which, consequently, does not easily admit an internal mo- 
tion, all its particles will retain their situation, and be electrified 
during: the whole time of the transmission. Let the uniform sub- 
O 
stance AG (PI. I. Fig. 8.) be supposed to connect an electrified 
body A with a vast mass at G. During the first instant, a mi- 
nute portion of the electricity is communicated to a certain dis- 
tance B, such, that the little space AB becomes electrified as 
intensely as the body A. In the second instant, the half of this 
portion will be distributed to an equal distance BC ; but the 
equilibrium which would thus be established through AC, is at 
the same moment destroyed, by a restoration of the electricity 
in AB. In the third instant, a smaller communication is made 
to the space CD, while BC receives a larger accession ; and thus 
