ten. Do not make the very common mistake of figuring that a stream is
delivering a cubic foot a minute to each square inch of weir opening,
simply because it _fills_ a certain opening. It is the excess water,
falling _over_ the opening, after the stream has set back to a
permanent dead level, that is to be measured.

This farmer who spends an idle day measuring the flow of his brook
with a notched board, may say here: "This is all very well. This is
the spring of the year, when my brook is flowing at high-water mark.
What am I going to do in the dry months of summer, when there are not
250 cubic feet of water escaping every minute?"

There are several answers to this question, which will be taken up in
detail in subsequent chapters. Here, let us say, even if this brook
does flow in sufficient volume only 8 months in a year--the dark
months, by the way,--is not electricity and the many benefits it
provides worth having eight months in the year? My garden provides
fresh vegetables four months a year. Because it withers and dies and
lies covered with snow during the winter, is that any reason why I
should not plow and manure and plant my garden when spring comes
again?

A water wheel, the modern turbine, is a circular fan with curved iron
blades, revolving in an iron case. Water, forced through the blades of
this fan by its own weight, causes the wheel to revolve on its axis;
and the fan, in turn causes a shaft fitted with pulleys to revolve.

The water, by giving the iron-bladed fan a turning movement as it
rushes through, imparts to it mechanical power. The shaft set in
motion by means of this mechanical power is, in turn, belted to the
pulley of a dynamo. This dynamo consists, first, of a shaft on which
is placed a spool, wound in a curious way, with many turns of
insulated copper wire. This spool revolves freely in an air space
surrounded by electric magnets. The spool does not touch these
magnets. It is so nicely balanced that the weight of a finger will
turn it. Yet, when it is revolved by water-power at a predetermined
speed--say 1,500 revolutions a minute--it generates electricity,
transforms the mechanical power of the water wheel into another form
of energy--a form of energy which can be carried for long distances on
copper wires, which can, by touching a button, be itself converted
into light, or heat, or back into mechanical energy again.

If two wires be led from opposite sides of this revolving spool, a