moisture--The barometer--The column barometer--The wheel
barometer--A very simple barometer--The aneroid
barometer--Barometers and weather--The diving-bell--The
diving-dress--Air-pumps--Pneumatic tyres--The air-gun--The
self-closing door-stop--The action of wind on oblique surfaces--The
balloon--The flying-machine.

When a child's rubber ball gets slack through a slight leakage of air,
and loses some of its bounce, it is a common practice to hold it for a
few minutes in front of the fire till it becomes temporarily taut again.
Why does the heat have this effect on the ball? No more air has been
forced into the ball. After perusing the chapter on the steam-engine the
reader will be able to supply the answer. "Because the molecules of air
dash about more vigorously among one another when the air is heated, and
by striking the inside of the ball with greater force put it in a state
of greater tension."

If we heat an open jar there is no pressure developed, since the air
simply expands and flows out of the neck. But the air that remains in
the jar, being less in quantity than when it was not yet heated, weighs
less, though occupying the same space as before. If we took a very thin
bladder and filled it with hot air it would therefore float in colder
air, proving that heated air, as we should expect, _tends to rise_. The
fire-balloon employs this principle, the air inside the bag being kept
artificially warm by a fire burning in some vessel attached below the
open neck of the bag.

Now, the sun shines with different degrees of heating power at different
parts of the world. Where its effect is greatest the air there is
hottest. We will suppose, for the sake of argument, that, at a certain
moment, the air envelope all round the globe is of equal temperature.
Suddenly the sun shines out and heats the air at a point, A, till it is
many degrees warmer than the surrounding air. The heated air expands,
rises, and spreads out above the cold air. But, as a given depth of warm
air has less weight than an equal depth of cold air, the cold air at
once begins to rush towards B and squeeze the rest of the warm air out.
We may therefore picture the atmosphere as made up of a number of
colder currents passing along the surface of the earth to replace warm
currents rising and spreading over the upper surface of the cold air. A
similar circulation takes place in a vessel of heated water (see p. 17).

LAND