As they pass from the hotter to the colder regions, a
number of particles coalesce; these again combine with others on the road
until the vapor becomes visible as cloud. The increased density implies
increased weight, and the cloud particles, as they sail poleward, descend
toward the surface of the earth. Assuming that a spherical form is
maintained throughout, the condensation of a number of particles implies a
considerable reduction of surface. Thus, the contents of two spheres vary
as the cubes of their radii, or eight (the cube of 2) drops on combining
will form a drop twice the radius of one of the original drops. We may
safely conceive hundreds and thousands of such combinations to take place
until a cloud mass is formed, in which the constituent parts are more or
less in contact, and, therefore, behave electrically as a single conductor
of irregular surface, upon which is accumulated all the electricity that
was previously distributed over the surfaces of the millions of particles
that now compose it.
The tendency of an electric charge upon the surface of a conductor is to
take upon itself a position in which it may approach nearest to an equal
and opposite charge; or, if possible, to attain neutrality. If, then, a
cloud has a charge, and there is no other cloud above or near it, the
charge _induces_ on the adjacent earth surface electricity of the opposite
kind.
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