On tlie Water Supply of Sydney. 139 



render this part at least perfectly impervious to percolation. 

 The lower portions would be sufficiently so as not to be apparent 

 by any appreciable waste, seeing there will at all times be a su- 

 perabundance of water for any emergency. 



The cost of this log-faced dam will be about the same as the 

 former rubble dam, i.e., £80,000, the extra amount required for 

 the log- facing, &c, being saved in the smaller waste weir re- 

 quired, this dam being also a waste weir, and passing about half 

 the flood waters ®ver it. The increased cost of the cofferdam 

 and outer log front, should it be required, will be about 

 £30,000 or £110,000 complete. The log dam will be similar to 

 that across the Eiver Schuylkill, which is an estuary of similar 

 width, and was erected in 1822 for supplying Philadelphia with 

 water. But this river has a much larger wLtershed and a far 

 more rapid stream than the George's Eiver, the former having a 

 fall of from 2 to 3 feet per mile, whereas the latter has only about 

 1-^ or 2 inches per mile, the .current alluded to in the report of 

 the Sydney Corporation on water supply being a tidal current 

 which will be done away with by the proposed dam. 



The log dam at Philadelphia is only erected upon a natural 

 ridge of rocks, which have a fall in the length of the dam of 24 

 feet ; and in sinking the log frames upon this ridge of rock it 

 was not even levelled or stopped as would be done in a dam of 

 masonry. The supply of water is also much larger than that 

 from the George's Eiver, as will be seen from the following 

 extract from ' Civil Engineering of North America, by David 

 Stevenson, F.E.S.E.': — 



" The Eairmount Waterworks are situate on the east bank of 

 the Eiver Schuylkill, about a mile and a half from the town of 

 Philadelphia. They are remarkable for their efficiency and sim- 

 plicity, as well as their great extent. They were commenced in 

 1819, and were in a working state in 1822. According to the 

 Water Company's report for the year 1836, the whole sum ex- 

 pended in their execution, up to that date, was £276,206. 



The water of the Eiver Schuylkill, with which the town of Phil- 

 adelphia is supplied, is raised by water power into four large 

 reservoirs, placed on a rocky eminence near the bank of the river ; 

 and after passing through the gravel filter beds, it is conveyed in 

 two large mains to the outskirts of the town, and thence led into 

 the various streets by smaller mains and branch-pipes. 



The erection of the dam across the river was the first and most 

 arduous part of the work. It measures about sixteen hundred 

 feet in length from bank to bank, and creates a stagnation in the 

 flow of the stream, which extends about six miles up the river. 

 The greatest depth of water in the line of the dam at low water 

 of spring tides is twenty-four feet, and the rise of tide is six feet, 



