DAM, TINKER'S, a guard of dough or clay placed by a tinker around a cavity to confine the melted metal until it'" sets." It is worthless after use.
DAM (Fr. barrage), a harrier for raising the level of water in a stream, for the purpose of forming a reservoir, or for turning the water in another direction. Several dams are sometimes placed upon a watercourse for the purpose of preventing too rapid an escape of water where it is needed for irrigation or for moving machinery. There is also a variety of dam called a coffer-dam, in which an inelosure is bounded by a barrier which prevents exterior water from entering, used generally for the purpose of excavation. Dams constructed for raising the level of water have an important use in the slack•water navigation of rivers. The materials which enter into the construction of dams differ according to circumstances. If the structure be required to bar a narrow gorge and a considerable stream, it must be made 'very strong, not only to withstand the hydrostatic pressure, but also the force of the current, which often, during fresh ets, becomes very great. The materials are then generally composed of a combination of wood-work and niasonry. Masonry may be principally used when the gorge is so narrow as to allow of the construction of a sufficiently small horizontal arc to resist the pressure. When the dam is very long (across a wide stream), unless a vast amount of stone is used, wooden races must be employed. Where the body of water to be restrained is not more than 4 or 5 ft. deep and the bottom is firm, a clay or stiff loam embankment 9, or 10 ft. thick, well compacted, will answer the purpose if a gate lie provided to keep the water from flowing over the top of the embankment, which would cause it to wear away. It is not always economy to build the darn in the narrowest part of a stream, or where the opposite banks nearest approach each other. This will often cause during a freshet too great a depth of running water over the dam, by which it may be endangered. A point should be selected where the dam can be made of suf ficient width to allow the water to pour over it without piling up too much, and where the foundation is good. The line of a darn may be transverse or diagonal to the flow of water. The diagonal is sometimes of advantage in increasing the width of flow, but is liable to interfere with the bed of the stream below more than the transverse line. Where practicable, the form of an arc, the convexity fronting up stream, is the best; but a broken line may sometimes be employed to advantage, the angles pointing up stream acting as braces, while the angles pointing down stream may be held by natural rock formation or heavy masonry, strengthened by bracing. There are a great many large dams in the manufacturing districts of New England, and in freshets, the giving way of some of them through faulty construction, has caused great destruction of life and property.
An example of a well-construeted dam is at Holyoke, Mass., across the Connecticut river. It is 1017 ft. long and 30 ft. high. The braces are formed of large square tim bers inclined 22° from the horizontal, with the lower ends bolted in the rock, and the upper ends sustaining timber frame-work. The canal for delivering the water, which is received by thirteen gates, is faced with masonry, and is 144 ft. wide at the top, and 22 ft. deep. It is said to be the best water motive-power utilized in the United States. A remarkable dam exists across the river Furens, in France, for protecting the town of St. Etienne front' freshets, and also for supplying the town with water. It is 164 ft. high, and 328 ft. wide at the top. The excavations for the foundation were very great and expensive, and the dam was constructed entirely of masonry, the stone laid not in tiers, but so as to produce a unity of mass, and with hydraulic cement, which is the mortar always used. The pressure of the contained water, at the depth of 154 ft., as much as 67 lbs. to the square inch, has sometimes been sufficient on this dam to force water through the pores of the material. In India, dams are constructed for purposes of irrigation, and some of them are of enormous magnitude. One of the longest is on the Godavery river at Dowlaisweram. Its total length Is 4,872 feet.
A good example of the mode of constructing a coffer-damn ander great difficul ties, on account of quicksand bottom, was furnished in the work preliminary to the building of the dry-dock at the Brooklyn navy-yard. There was over 60 ft. of utterly unstable micaceous sand below the mud at the bottom of the river. This of course, under so great pressure, would flow almost like water itself. The area required to be excavated through this material was over two acres at the top and one acre at the bottom, which was 37 ft. below mean high water. The first attempt was a failure, and longer and stronger piles were then used, filled between with stone and coarse gravel. There were six concentric rows of piles, the walls being over 60 ft. thick.
Where the bottom does not admit of pile-driving, crib-work, weighted with stone and sunk in proper position, is used, the crevices being stopped with hydraulic cement. At Blossom Rock, in San Francisco harbor, a combination of crib-work and iron cylinder was used in the construction of the coffer-darn by means of which the excavations were made, preparatory to blasting. An iron cylinder 6 ft. in diameter, armed with thick india-rubber flaps at the bottom, was sunk to the ground and then surrounded with crib-work. Excavation was then made within the cylinder, which was from time to time depressed until the depth was reached necessary to exclude the water.