ind a
wide board a little longer than the width of this creek you have
scorned. Set it upright across the stream between the banks, so that
no water flows around the ends or under it. It should be high enough
to set the water back to a dead level for a few feet upstream, before
it overflows. Cut a gate in this board, say three feet wide and ten
inches deep, or according to the size of a stream. Cut this gate from
the top, so that all the water of the stream will flow through the
opening, and still maintain a level for several feet back of the
board.

This is what engineers call a weir, a handy contrivance for measuring
the flow of small streams. Experts have figured out an elaborate
system of tables as to weirs. All we need to do now, in this rough
survey, is to figure out the number of square inches of water flowing
through this opening and falling on the other side. With a rule,
measure the depth of the overflowing water, from the bottom of the
opening to the top of the dead level of the water behind the board.
Multiply this depth by the width of the opening, which will give the
square inches of water escaping. For every square inch of this water
escaping, engineers tell us that stream is capable of delivering,
roughly, one cubic foot of water a minute.

Thus, if the water is 8 inches deep in an opening 32 inches wide, then
the number of cubic feet this stream is delivering each minute is 8
times 32, or 256 cubic feet a minute. So, a stream 32 inches wide,
with a uniform depth of 8 inches running through our weir is capable
of supplying the demands of the average farm in terms of electricity.
Providing, of course, that the lay of the land is such that this water
can be made to fall 10 feet into a water wheel.

Go upstream and make a rough survey of the fall. In the majority of
instances (unless this is some sluggish stream in a flat prairie) it
will be found feasible to divert the stream from its main channel by
means of a race--an artificial channel--and to convey it to a not
far-distant spot where the necessary fall can be had at an angle of
about 30 degrees from horizontal.

If you find there is _twice_ as much water as you need for the amount
of power you require, a five-foot fall will give the same result. Or,
if there is only _one-half_ as much water as the 250 cubic feet
specified, you can still obtain your theoretical five horsepower if
the means are at hand for providing a fall of twenty feet instead of