, Pennsylvania, several years ago. If it
is possible to carry the overflow water of the stream away in some other
channel than over the dam, then a dirt dam is not objectionable,
although always a dirt dam is best with a masonry core. A very good dam
can be made by driving three-inch tongue-and-grooved planking tight
together across a gulley and then filling in on each side so that the
slope on each face is at least two feet horizontal for every foot in
height. This last requirement means that if the dam is ten feet high,
the width of the dam at the base shall be at least forty-five feet, the
other five feet being required to give the proper thickness to the dam
at the top.

[Illustration: FIG. 39.--Concrete core in a dam.]

In the second type of dam this central timber core is replaced with a
thin wall of concrete as shown in Fig. 39, from six to twelve inches
thick, sufficing to prevent small animals burrowing through the dam and
at the same time to make the dam more nearly water-tight. Sometimes
stone masonry is used, building a light wall to serve as the true dam,
and then holding up this light wall with earth-filling on each side. If
neither plank, stone, nor concrete can be used, the central core is
made of the best earth available, a mixture of clay and sand preferably,
and special pains are taken in the building to have this mixture well
rammed and compacted.

The writer has recently heard of a dam on a small stream being made by
the continual dumping of field stone from the farm into the brook at a
certain definite place. This stone, of course, assumed a slope at each
side and settled in place from year to year as the dam grew. The mud and
silt of the stream filled up the holes between the stones, so that the
dam was finally practically water-tight. This made a cheap construction
and had the additional value of serving to use up stones from the
fields. It was necessary, since the spring floods poured over the top of
this dam, to protect the top stones, and a plank crest was put on,
merely to keep the dam from being washed away.

The third type of dam is entirely of concrete or stone masonry, concrete
to-day being preferable because more likely to be water-tight. The
problem with a concrete dam is to get a foundation such that the
impounded water will not leak out under the dam, imperiling the very
existence of it. The ideal foundation, of course, is rock, and in a
great many locations can be found in the sm