ably by an opening in one corner so arranged as to carry
the side walls of the opening or manhole up above the ground, where it
may be protected with an iron cover locked fast (see Fig. 38, after
Imbeaux). Besides the outlet pipe from the spring, which will naturally
pass through the side walls about halfway between top and bottom in
order to get the best water, there should be a drainpipe from the lowest
part of the inclosure, the valve of which can be reached through a valve
box coming to the surface. In the figure the drainpipe is shown by the
dotted line, and the twofold chamber is for the purpose of allowing an
examination of the spring to be made at any time.

The concrete used in this work should be of good quality, one part of
cement to five parts of gravel or to four parts of stone and two parts
of sand. A concrete bottom, although sometimes used, is not necessary.
The position of the drain, of the house pipe, and of the several
collection pipes must not be overlooked when the wall is being built,
since it is much easier to leave a hole than to dig through the concrete
afterwards.

_Stream supplies._

If the volume of a stream is more than enough for the maximum
consumption, nothing is needed but to carry the intake pipe from the
shore out under water and protect the end with a strainer. In this case,
however, the stream may freeze down to the level of the strainer and
even around the strainer, so that the supply of water in winter would be
cut off. To avoid this possibility the intake pipe ought to be in a pool
of water so deep that it never freezes, and this means sometimes
creating a pool for this very purpose. If storage is to be provided, a
reservoir must be built, and this intake pipe would naturally be placed
at least two feet below the surface of the water.

_Dams._

If the stream is not deep, or if there is not a pool of satisfactory
depth, or if the minimum flow of the stream is not adequate for the
maximum needs of the consumers, a dam across the stream becomes a
necessity. There are two or three types of dams suitable for a reservoir
on a small stream, and they may be described briefly.

A dirt dam is not generally desirable, since in most cases the dam must
also be used as a waste weir; that is, the freshets must run over the
dam. This means that unless the crest of the dam is protected with
timber or masonry the dam will be washed out; as happened, indeed, in
the terrible flood at Johnstown