is filled by an upper current flowing back from the
equator; and this goes on continually, and keeps the great atmospheric
wheel turning. Wherever a wind blows, the process is similar: it is the
sun that causes the wind, be it zephyr, or gale, or hurricane.

"But," you will say, "the sun does not run our steam-engines; it is
artificial heat, not natural heat, that changes the water into steam."
Very true; but how do we get this heat? By burning wood or coal. For the
former we are clearly in debt to the sun, which made the trees grow that
furnish the fuel; and the coal is the remains of plants that grew long
before the creation of man, plants that were as dependent on the
sunshine as those that flourish to-day. When we burn coal, the heat we
get from it is nothing but the sunbeams that were caught and imprisoned
by those ancient plants; our steam-engines use the force that was stored
up by the sun millions of years before the steam-engine was invented.

All muscular power, whether of man or of other animals, may be traced to
the same source. Animals get their food either from plants or from other
animals that have fed upon plants; and the plants owe their existence to
the sun. The animal is a machine, like the steam-engine; the food which
it eats is the fuel that keeps the machine in action. With every
movement we make, a portion of this fuel is burned up in our muscles.
Every beat of our hearts is at the expense of such material; and the
material is the gift of the sun. Our very thoughts are indirectly
dependent on the sunbeams; for the brain, which is the organ of thought,
requires food to maintain its activity, like the muscles and all the
other machinery of the body.

There are other kinds of force less familiar than these--as electricity,
magnetism, and chemical force--which can also be proved to come
indirectly from the sun, but the proof can not be given here. We can
detect the work of the sunbeams in the flash of the lightning and the
roar of the thunder, in the turning of the compass-needle to the north,
and in all the wonders of chemical science, as certainly as in the
growing plant or the running stream.

The only form of force known to us which does not come entirely from the
sun is that of the tides. The tidal wave is raised and carried round the
earth mainly by the attraction of the moon. The sun, though immensely
larger than the moon, is so much farther off that it attracts the waters
of the earth much