Nevertheless, the finest cloud
or mist that ever existed consists simply of little globules of water
suspended in air, and thus for our present purpose differs in no
important respect from fog, dust, and smoke. A cloud or mist is, in
fact, fine water-dust. Rain is coarse water-dust formed by the
aggregation of smaller globules, and varying in fineness from the
Scotch mist to the tropical deluge. It has often been asked how it is
that clouds and mists are able to float about when water is so much
heavier (800 times heavier) than air. The answer to this is easy. It
depends on the resistance or viscosity of fluids, and on the smallness
of the particles concerned. Bodies falling far through fluids acquire
a "terminal velocity," at which they are in stable equilibrium--their
weight being exactly equal to the resistance--and this terminal
velocity is greater for large particles than for small; consequently
we have all sorts of rain velocity, depending on the size of the
drops; and large particles of dust settle more quickly than small.
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