mortar. The total amount of masonry is 325,000 cubic yards. There are two weirs, at a level of 6 ft. below the crest, each 82 ft. wide. The total height, including the foundations, which are carried down from 3 ft. to 5 ft. into the rock, is 154 ft., and the breadth of the crest, which carries a road, is 49 ft. 3 in., and at the base 216 ft. The outlet pipes are carried through tunnels, which are driven on the curve into the hill side a considerable distance clear of each end of the dam.

Another very important structure is the Villar dam, Fig. 15, in connection with the water supply of Madrid, and situated on the river Lozoya. The storage capacity of this reservoir is very considerable, viz., 4,400,000,000, or nearly thirteen times as great as that of Furens. The height of the dam is 162 ft., with a breadth of 14 ft. 9 in. at the crest. It is built on the curve to a radius of 440 ft., and the length of the dam measured along the crest is 546 ft., of which 197 ft. is by-wash, thus describing nearly one-fifth of a circle, and consequently well designed to resist pressure. The dam is built of rubble masonry in hydraulic mortar, and cost £80,556.

The Stony Creek lower reservoir dam of the Geelong water supply, Fig. 16, colony of Victoria, is interesting as being constructed of concrete, in the proportion of 1 to 8½. Its erection occupied eighteen months, and cost about £18,000. It is curved in plan to a radius of 300 ft., and the greatest depth or head of water is 52 ft. 4 in. The width at the crest is only 2 ft. 8 in., although surmounted by a heavy coping of bluestone 3 ft. 3 in. broad and 1 ft. 9 in. deep. There being no facility for making a by-wash at the side, the center of the dam is dished to form a weir 30 ft. long. There are both outlet and scour pipes, and valves of 2 ft. diameter, and the capacity of the reservoir is 143,145,834 gallons.

The Paramatta dam, in New South Wales, built of masonry in hydraulic mortar, is another instance of a dam built on the curve, and which has resisted a flood of water 4 ft. in depth over the crest; and in the case of a dam of about 40 ft. high across the river Wyre, in connection with the Lancaster Water Works, made of cement concrete in proportion of 4 to 1, there has, according to Mr. Mansergh, frequently been a depth of 5 ft. of flow over it. This dam is built to a radius of 80 ft. only, and as it measures 100 ft. along the crest, must include about the fifth of a circle.

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 2½ 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 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.

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.

A diagram comparing the section derived from Molesworth's formula and those of Furens, Gileppe, Vyrnwy, and Quaker Bridge, is given at Fig. 20, the limit of pressure assumed for the masonry being 93 lb. per square inch, which is that of the Furens, the Gileppe being 88.