square miles of watershed by thirty. Then
an area of twelve square miles would need a length of waste channel of
three hundred sixty feet; in most cases, for small dams, longer than the
dam itself.

If the depth be taken at two feet, then the number of square miles of
watershed must be multiplied by ten to get the length of weir, so that a
shed of twelve square miles would mean a weir one hundred twenty feet
long.

The factor for a depth of three feet on the weir is six, making for the
same area the length of weir seventy-two feet, and for four feet depth
the factor is four. There is no more important part of the construction
of a dam than that involved by a proper design of a waste weir, since a
failure either to provide proper area or to so build as to withstand the
erosive action of the running water will inevitably wash away the dam.

When the valley is narrow and the watershed large, the waste weir will
occupy the entire width of the dam, and then it becomes necessary to
construct the dam in masonry. On the other hand, when the watershed is
small and the width of the valley great, then it is proper to make the
waste weir only a certain portion of the entire width of the dam, making
the rest of the dam either masonry or earth, as may be convenient.

_Gate house._

In connection with a reservoir and at the back of the dam at the bottom
of the bank, it is convenient to have what is called, in larger
installations, a "gate house"; that is, a masonry or wooden manhole
through which the water-pipe leading out from the reservoir passes and
in which a gate is placed to shut off the water. In larger
installations, it is usually possible to admit water at this point from
different levels of the reservoir into the water-pipe, so as always to
get the best quality of water, but for a small plant that is not
necessary. A gate or valve, however, should always be provided, and
while this may be on the bank of the pond with the intake pipe extending
twenty or thirty feet into the pond, the valve should not be omitted.
The end of the pipe extending into the pond should be placed about two
feet above the bottom of the pond, instead of resting in the mud, in
order to get a better quality of water.

_Pipe lines._

In bringing the water from the spring or pond to the house, some kind of
a pipe line must be provided. Such a pipe line is made of various
materials; hollow wooden logs, vitrified tile, cast-iron pipe,
wrought-iron