inches wide and 6-1/4 deep, giving 189 cubic feet of water a
minute--_Quantity._ Since such a stream is of common occurrence on
thousands of farms, let us analyze briefly its possibilities for
power: One hundred and eighty-nine cubic feet of water weighs 189 x
62.5 pounds = 11,812.5 pounds. Drop this weight one foot, and we have
11,812.5 foot-pounds. Drop it 3 feet and we have 11,812 x 3 =
35,437.5 foot-pounds. Since 33,000 foot-pounds exerted in one minute
is one horsepower, we have here a little more than one horsepower. For
simplicity let us call it a horsepower.

[Illustration: Detail of a water-power plant, showing setting of
wheel, and dynamo connection]

Now, since the work to be had from this water varies directly with
_quantity_ and _head_, it is obvious that a stream _one-half_ as big
falling _twice_ as far, would still give one horsepower at the wheel;
or, a stream of 189 cubic feet a minute falling _ten times_ as far, 30
feet, would give _ten times_ the power, or _ten_ horsepower; a stream
falling _one hundred times_ as far would give _one hundred_
horsepower. Thus small quantities of water falling great distances, or
large quantities of water falling small distances may accomplish the
same results. From this it will be seen, that the simple formula for
determining the theoretical horsepower of any stream, in which
Quantity and Head are known, is as follows:

Cu. Ft.
per Feet
minute x head x 62.5
(A) Theoretical Horsepower = ----------------------
33,000

_As an example, let us say that we have a stream whose weir
measurement shows it capable of delivering 376 cubic feet a minute,
with a head (determined by survey) of 13 feet 6 inches. What is the
horsepower of this stream?_

Answer:

Cu. ft. p. m. head pounds
376 x 13.5 x 62.5
H.P. = ----------------------------- = 9.614 horsepower
33,000

This is _theoretical horsepower_. To determine the _actual_ horsepower
that can be counted on, in practice, it is customary, with small water
wheels, to figure 25 per cent loss through friction, etc. In this
instance, the actual horsepower would then be 7.2.

_The Size of the Wheel_

Water wheels are not rated by horsepower by manufacturers, because the
same wheel might develop one horsepower or one hundred hor