r" in one mass where it would do the most good. The farmer
who makes use of the energy of falling water to generate electricity
for light, heat, and power does the same thing--he makes use of the
capacity for work stored in water in being lifted to a certain height.
As in the case of the gasoline engine, which burns 14 pounds of air
for every pound of gasoline, the engineer of the water-power plant
does not have to concern himself with the question of how this
natural source of energy happened to be in a handy place for him to
make use of it.

The sun, shining on the ocean, and turning water into vapor by its
heat has already lifted it up for him. This vapor floating in the air
and blown about by winds, becomes chilled from one cause or another,
gives up its heat, turns back into water, and falls as rain. This
rain, falling on land five, ten, a hundred, a thousand, or ten
thousand feet above the sea level, begins to run back to the sea,
picking out the easiest road and cutting a channel that we call a
brook, a stream, or a river. Our farm lands are covered to an average
depth of about three feet a year with water, every gallon of which has
stored in it the energy expended by the heat of the sun in lifting it
to the height where it is found.

The farmer, prospecting on his land for water-power, locates a spot on
a stream which he calls Supply; and another spot a few feet down hill
near the same stream, which he calls Power. Every gallon of water that
falls between these two points, and is made to escape through the
revolving blades of a water wheel is capable of work in terms of
foot-pounds--an amount of work that is directly proportional to the
_quantity_ of water, and to the _distance_ in feet which it falls to
reach the wheel--_pounds_ and _feet_.

_The Efficient Water Wheel_

And it is a very efficient form of work, too. In fact it is one of the
most efficient forms of mechanical energy known--and one of the
easiest controlled. A modern water wheel uses 85 per cent of the total
capacity for work imparted to falling water by gravity, and delivers
it as rotary motion. Compare this water wheel efficiency with other
forms of mechanical power in common use: Whereas a water wheel uses 85
per cent of the energy of its water supply, and wastes only 15 per
cent, a gasoline engine reverses the table, and delivers only 15 per
cent of the energy in gasoline and wastes 85 per cent--and it is
rather a high-class gasoline en