erent
temperatures. A current of cold air meeting a current of warm, moist
air in its course may condense a considerable portion of the moisture
into clouds and rain, and this condensation will go on as long as the
currents continue to meet. In a hot spring day a mass of air which has
been warmed by the sun, and moistened by evaporation near the surface
of the earth, may rise up and cool by expansion to near the
freezing-point. The resulting condensation of the moisture may then
produce a shower or thunder-squall. But the formation of clouds in a
clear sky without motion of the air or change in the temperature of the
vapor is simply impossible. We know by abundant experiments that a mass
of true aqueous vapor will never condense into clouds or drops so long
as its temperature and the pressure of the air upon it remain unchanged.

Now let us consider sound as an agent for changing the state of things
in the air. It is one of the commonest and simplest agencies in the
world, which we can experiment upon without difficulty. 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. This notion, I believe, is not confirmed by statistics; but,
whether it is or not, we can say with confidence th