s all,
or nearly all, the work of the world. We talk of water-power,
wind-power, steam-power, animal power, and the like; but all these are
only kinds of sun-power. Let us look at them one by one, and see if the
sunbeams are not the forces within or behind them all.

Water-power is the force exerted by falling or running water; and
running water is falling water. In the most familiar forms of
water-wheels, troughs--or buckets, as they are called--are arranged on
the rim in such a way that the water runs into those on one side of the
wheel near the top, making that side heavier, so that it descends. As
the buckets go down, the water runs out of them, but those above are
being filled in their turn, so that this side of the wheel is
continually weighted with water, while on the other side empty buckets
are going up. The wheel may turn mill-stones to grind wheat or corn, or
may give motion to machinery for spinning and weaving cotton or wool;
but is it the water-wheel that really does the work? "No," you will say;
"if we trace back the force that moves the machinery, we find it in the
falling water that fills the buckets of the wheel; it is the water-fall
that is the real worker." No; it is the sun, which is a force behind the
water-fall, as the water-fall is the force behind the wheel. What
supplies the water-fall with its never-failing stream? The rain that
fills the springs high up among the hills, where a little brook has its
source--the rain that feeds the brook as it flows, and other brooks that
join it on its way, until it becomes the river that descends in the
water-fall. And what is the source of the rain? The sun, whose rays turn
the waters of the earth to vapor, and lift them up to the clouds, whence
they fall upon the hills. Were it not for the sun the rain would soon
cease to fall, the springs in the hills would dry up, the brooks would
run out, the river would dwindle away, the roar of the water-fall would
die into silence, and the wheel would stop for want of power.

The wind, which is the motive force of windmills and of sailing vessels,
is another form of sun-power. The atmosphere has been compared to a
great wheel carried round by the heat of the sun. We know that when air
is heated it rises, and that the tropical parts of the earth are hotter
than the polar regions. In the tropics, therefore, the heated air rises,
and the colder air from the poles flows in to fill its place, while the
place of the latter