It is purely mechanical in its action. When a bomb
explodes, a certain quantity of gas, say five or six cubic yards,
is suddenly produced. It pushes aside and compresses the
surrounding air in all directions, and this motion and compression
are transmitted from one portion of the air to another. The amount
of motion diminishes as the square of the distance; a simple
calculation shows that at a quarter of a mile from the point of
explosion it would not be one ten-thousandth of an inch. The
condensation is only momentary; it may last the hundredth or the
thousandth of a second, according to the suddenness and violence
of the explosion; then elasticity restores the air to its original
condition and everything is just as it was before the explosion. A
thousand detonations can produce no more effect upon the air, or
upon the watery vapor in it, than a thousand rebounds of a small
boy's rubber ball would produce upon a stonewall. So far as the
compression of the air could produce even a momentary effect, it
would be to prevent rather than to cause condensation of its
vapor, because it is productive of heat, which produces
evaporation, not condensation.
The popular notion that sound may produce rain is founded
principally upon the supposed fact that great battles have been
followed by heavy rains.
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