138 On the Water Supply of Sydney. 



the vested interests of proprietors would not be interfered with, 

 in excess of the money saved by this simple mode of construction, 

 it might be advisable to raise the dam and waste weir to such a 

 height as always to keep the fresh or impounded water above 

 the sea water. To do this it will be necessary to provide storage 

 for a year's supply, or about 600 million cubic feet to meet any 

 possible drought that might occur. Even to do this it will be 

 only necessary to raise the dam about 3 feet higher, or 11 feet 

 above low water : we should then make sure of having the pres- 

 sure on the right side to exclude the sea or salt water, and would 

 only have to throw in loose material, such as clay, should the 

 floods not bring down sufficient mud or clay for the purpose ; but 

 as the river bed is partly formed of clay, such material has evi- 

 dently been, and is doubtless still, brought down by every flood, 

 so that little need he apprehended on this point should vested 

 interests permit of such a simple and natural dam being made 

 for so desirable an object. The cost of raising the dam the in- 

 creased height proposed would not be enhanced, as the extra 

 material required for the higher dam must be removed to form 

 the waste weir or new channel of river. In either case, it would 

 cost about £80,000, wet-top or waste weir dam. If it should be 

 found advisable, on account of vested interests, not to raise the 

 river more than 2^ feet above high water of spring tides, and 

 also dispose of the flood waters at a height of about 12 inches 

 above waste weir ; then a wet-top dam might be constructed (as 

 shewn by drawing) after the American system, where, in the 

 place of a retaining or breast wall of masonry, they bolt round 

 logs together, and sink them with stones, in portions or sections 

 of about 20 feet square. These log frames could be partly sea- 

 soned before being launched, from timber obtained on the banks 

 of the river above the proposed dam, or on the sea coast, and 

 floated to Kangaroo Point; the front tier being strung to iron 

 rods with chisel points, jumped into the rock bed of river, to get 

 them in line, so that they would form a vertical staging from 

 which to construct (hereafter if required) a cofferdam, with iron 

 rods for guide piles, and horizontal log sheeting piles strung on 

 rods, as shewn on drawing. As there would be a pressure of 

 2000 to 3,600 lbs. on the square foot on the lower half of the 

 dam, it would be impossible to pump the water out of the coffer- 

 dam, the bottom having been previously dredged down to the 

 rock. Portland cement beton would have to be introduced in 

 bags in a semi-fluid state, so that they could adapt themselves to 

 the inequalities of the bottom ; or large bags of the width of the 

 cofferdam, and of any convenient length, might be lowered and 

 fitted to the bottom by a diver, who would be required in either 

 case for the lowest stratum of beton ; the upper portions of the 

 cofferdam might be pumped out as far as practicable so as to 



