ak, but not so as to endanger the dam. In
nine tenths of the cases, the dam is breached along the line of the water
outlet passages."

The method of forming the discharge outlet by the construction of a
masonry culvert in the open has no doubt many advantages over that of
tunnel driving through the hill side clear of the dam, permitting as it
does of an easy inspection and control of the work as it proceeds; but a
slight leakage in the instance of a side tunnel probably means nothing
more than the waste of so much water, whereas in the case of the culvert
traversing the site of the bank, the same amount or less imperils the
stability of the bank, and in ninety-nine cases out of a hundred would,
if not attended to, sooner or later be the cause of its destruction. I
think the majority will therefore agree that the method of discharge
outlets under the site of embankments should not be tolerated where it is
possible to make an outlet in the flank of the hill, to one side, and
altogether clear of the dam.

At Fig. 9 is a diagram of the Roundwood dam of the Vartry Water Works,
supplying Dublin, which is a fair specimen of the class of earthwork dam
with the outlet pipes carried in a culvert under the embankment, and
which, perhaps, is one of the most favorable specimens of this method of
construction, as the inlet valves are on the up stream of the dam, and
consequently when necessary the water can be cut off from the length of
pipes traversing the dam. A short description will be given. This dam is
66 ft. high at the deepest point and 28 ft. wide at the crest, having to
carry a public road. The slope on the inner face is 3 to 1, and on the
outer 21/2 to 1. The by-wash is 6 ft. below the crest, which is about the
average difference. The storage capacity of the reservoir is
2,400,000,000 gallons, or sufficient for 200 days' supply to the city.
The puddle wall is 6 ft. wide at the top and 18 ft. at ground level, the
bottom of the puddle trench about 40 ft. below the surface of the ground.
The culvert was formed by cutting a gullet 14 ft. wide with nearly
vertical sides through the rock, and covering it with a semicircular arch
4 ft. in thickness. Through this tunnel are laid a 33 in. and 48 in.
main; the former for the water supply, and the latter for scouring or for
emptying the reservoir on an emergency. There is a plugging of brickwork
in cement under the center of the dam in the line of the puddle wall, and
then stop wall