There now remain only two other examples of masonry dams, the first being that in connection with the Liverpool water supply, and known as the Vyrnwy dam, Fig. 17, this being thrown across a stream of that name in North Wales.  It is now under construction, and when completed will impound an area of 1,115 acres.

The dam will be 1,255 ft. long, and formed of Cyclopean rubble set in cement mortar, and the interstices or spaces between the large masses of stone, which are rough hewn and not squared, are filled with cement concrete.  The proportion of the cement mortar is 21/2 to 1.  These masses of stone weigh from two to eight tons each, and it is expected that the wall will be of a most solid description, as great care is being taken to fill up all spaces.  The face next to the water is cemented.  The area of the cross section shown on the diagram, which is at one of the deepest points, is 8,972 square feet, and the height from foundation to flood level is 129 ft., the breadth at the base being 117 ft. 9 in.

The existing dam of the New York water supply, Fig. 18, known as the 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 21/2 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 L500,000.

[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.