140 On the Water Supply of Sydney. 



The bottom of the river consists of rock covered with a deposit of 

 mud 11 feet in depth, and the bottom is entirely composed of 

 bare rock, part of which, at the western side of the river, is ex- 

 posed during low water. The line of the dam forms an angle of 

 about 45 degrees with the direction of the stream. In this way 

 a large overfall is formed for the water, and its perpendicular 

 rise above the top of the dam, when the river is is a flooded state, 

 is not so great as it would have been had the dam been placed at 

 right angles to the stream. By adopting this direction the 

 strength of the structure is also considerably increased, for the 

 mass of the dam opposed to any given section of the stream is 

 greater directly as the cosine, or inversely, as the sine of the 

 angle formed by the line of the dam and the direction of the 

 stream impinging on it. 



The part of the dam which was first formed is that which is 

 founded on the mud bottom. It consists of a large mound com- 

 posed of rubble stones and earth thrown into the river. It mea- 

 sures 270 feet in length, 150 feet in breadth at the base, and 12 

 feet at the top, and its upper slope or face, which is exposed to 

 the wash of the river, is cased with rough pitching formed of 

 large stones. The termination of the dam is protected by a cut- 

 stone pier, measuring twenty-eight feet by twenty-three feet, 

 which is founded on rock, and built in water twenty-eight feet in 

 depth. 



The overfall dam measures 1204 feet in length, and is founded 

 on a rocky bottom, which rises pretty regularly from where there 

 is a depth of twenty-four feet during the lowest tides, towards 

 where the rock is uncovered at low water. 



The current of the river being strong, it was found impossible 

 to form this part of the dam by constructing a mouud of rubble 

 on the rocky bottom, according to the plan followed in founding 

 the first part of the structure, on a bottom composed of mud. 

 The expedient resorted to for retaining the stones on the shelving 

 rock was extremely ingenious, and has proved very effective. 



The overfall dara consists of a strong wooden framework or 

 crib, which was formed in separate compartments, and sunk in 

 small portions in the line of the dam, by filling it with stones. 

 The cribs are formed of logs of wood, measuring eighteen by 

 twenty inches, connected together by strong dove-tailing, notched 

 three inches deep. The size of the wooden frame-work measured, 

 in the direction of the stream is seventy-two feet, and the separate 

 compartments of which it was formed measured twenty feet in 

 breadth. The part of the dam over which the water flows, and 

 also the posterior part of it, are covered with planking six inches 

 in thickness. In forming the dam, the cribs were floated one 

 after another to the site which they were to occupy, and large 

 stones being thrown into them, they gradually sank, until at last 



