day running water is used as an
indirect source of electric lights for street and house, the energy of
the falling water serving to rotate the armature of a dynamo (Section
310).

[Illustration: FIG. 119.--The windmill pumps water into the troughs
where cattle drink.]

A more constant source of energy is that available from the burning of
fuel, such as coal and oil. The former is the source of energy in
locomotives, the latter in most automobiles.

In the following Chapter will be given an account of water, wind, and
fuel as machine feeders.

CHAPTER XVII

THE POWER BEHIND THE ENGINE

171. Small boys soon learn the power of running water; swimming or
rowing downstream is easy, while swimming or rowing against the
current is difficult, and the swifter the water, the easier the one
and the more difficult the other; the river assists or opposes us as
we go with it or against it. The water of a quiet pool or of a gentle
stream cannot do work, but water which is plunging over a precipice or
dam, or is flowing down steep slopes, may be made to saw wood, grind
our corn, light our streets, run our electric cars, etc. A waterfall,
or a rapid stream, is a great asset to any community, and for this
reason should be carefully guarded. Water power is as great a source
of wealth as a coal bed or a gold mine.

The most tremendous waterfall in our country is Niagara Falls, which
every minute hurls millions of gallons of water down a 163-foot
precipice. The energy possessed by such an enormous quantity of water
flowing at such a tremendous speed is almost beyond everyday
comprehension, and would suffice to run the engines of many cities far
and near. Numerous attempts to buy from the United States the right to
utilize some of this apparently wasted energy have been made by
various commercial companies. It is fortunate that these negotiations
have been largely fruitless, because much deviation of the water for
commercial uses and the installation of machinery in the vicinity of
the famous falls would greatly detract from the beauty of this
world-known scene, and would rob our country of a natural beauty
unequaled elsewhere.

[Illustration: FIG. 120.--A mountain stream turns the wheels of the
mill.]

172. Water Wheels. In Figure 120 the water of a small but rapid
mountain stream is made to rotate a large wheel, which in turn
communicates its motion through belts to a distant sawmill or grinder.
In more level regions