through the keyhole, and moans as it rushes down the valley.
What makes the air restless? why should it not lie still all
round the earth?

It is restless because, as you will remember, its atoms are kept
pressed together near the earth by the weight of the air above,
and they take every opportunity, when they can find more room, to
spread out violently and rush into the vacant space, and this
rush we call a wind.

Imagine a great number of active schoolboys all crowded into a
room till they can scarcely move their arms and legs for the
crush, and then suppose all at once a large door is opened. Will
they not all come tumbling out pell-mell, one over the other,
into the hall beyond, so that if you stood in their way you would
most likely be knocked down? Well, just this happens to the air-
atoms; when they find a space before them into which they can
rush, they come on helter-skelter, with such force that you have
great difficulty in standing against them, and catch hold of
something to support you for fear you should be blown down.

But how come they to find any empty space to receive them? To
answer this we must go back again to our little active invisible
fairies the sunbeams. When the sun-waves come pouring down upon
the earth they pass through the air almost without heating it.
But not so with the ground; there they pass down only a short
distance and then are thrown back again. And when these sun-
waves come quivering back they force the atoms of the air near
the earth apart and make it lighter; so that the air close to the
surface of the heated ground becomes less heavy than the air