Croton reservoir, is shown on the diagram. Its capacity is 364,000,000
gallons and the area 279 acres. The height is 78 ft. and width at crest 8
ft. 6 in., and is built of masonry in hydraulic mortar. The face walls
are of stone laid in courses of 14 in. to 26 in., and are vertical on the
up stream side, and with a batter of 1 in 2½ on the down. The hearting is
of concrete for a depth of 45 ft. from the top, and the remaining depth
is in Cyclopean rubble.

At Fig. 19 is shown the section of the Quaker Bridge dam, which when
completed will be the largest structure of the kind in existence. It is
situated on the Croton River, which is a tributary of the Hudson, about
four miles below the present Croton dam. The length will be 1,300 ft. and
the height 170 ft. above the river bed, or 277 ft. above the foundation.
The water by-wash is 7 ft. below the crest, and the dam is 26 ft. broad
at the crest and 216 ft. at the base. The capacity of the reservoir will
be 32,000,000,000 gallons, or nearly a hundred times as great as that of
Furens. The geological formation at the site is sienitic gneiss. The cost
of the dam is estimated at £500,000.

[Illustration: TYPICAL MASONRY AND EARTHWORK DAMS OF THE WORLD.]

The accompanying table gives the pressures to which various dams are
subjected, and it may be noted with regard to the weight of water,
generally assumed as 62.4 lb. per cubic foot, that it will, in some
districts, in time of flood, carry so much matter in suspension as to be
increased to as much as 75 lb. weight, or an addition of 20 per cent.,
which, it may be easily imagined, will affect the conditions of stability
very seriously.

TABLE OF MAXIMUM PRESSURES.