form of the base of the puddle trench--that instead of cutting the bottom
of the trench at the sides of the valley in steps, it should be merely
sloped, so that the puddle, in setting, tends to slide down each inclined
plane toward the bottom of valley, thereby becoming further compressed;
whereas, should the natural ground be cut in steps, the puddle in setting
tends to bulge at the side of each riser, as it may be termed, and so
cause fissures. It will be noticed that the slopes of these earthwork
dams vary from 7 to 1 to 2 to 1.

The depths to which some puddle trenches are carried has been objected to
by some engineers, and among them Sir Robert Rawlinson, as excessive and
unnecessary, and, in the opinion of the latter, the same end might be
obtained by going down to a depth say of 30 ft. only, and putting in a
thick bed of concrete, and also carrying up the concrete at the back of
the puddle trench, with a well for collecting water, and a pipe leading
the same off through the back of the dam to the down stream side. An
arrangement of this kind is shown in the Yarrow dam, Fig. 4.

The thickness of the puddle wall varies considerably in the different
examples given in the diagrams before you, a fair average being the Row
bank of the Paisley Water Works, Fig. 6; and although in instances of
dams made early in the century, such as the Glencorse dam--Fig. 5--of the
Edinburgh Water Works, the puddle was of very considerable thickness, and
it would appear rightly so. This practice does not seem to have been
followed in many cases, as, for instance, again referring to the Dale
Dyke dam, Fig. 2, where the thickness of the top was only 4 ft., with a
batter of 1 in 16 downward, giving a thickness of 16 ft. at the base. For
a dam 95 ft. in height this is very light, compared with that of the
Vehui dam at Bombay, of which the engineer was Mr. Conybeare--Fig.
7--where the puddle wall is 10 ft. wide at the top, with a batter