compression cannot take place in the air. The cooling which
results in the formation of clouds and rain may come in two ways.
Rains which last for several hours or days are generally produced
by the intermixture of currents of air of different 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