On the Water Supply of Sydney. 141 



they rested on the bottom of the river. The upper parts of the 

 several cribs, or those portions of them which stood above the 

 level of low water, were then firmly connected together, so as to 

 form one continuous framework, behind which a large mass of 

 rubble hearthing and earth was placed, to give the whole structure 

 weight and stability, and to prevent leakage. 



This mode of forming dams is very generally practised in 

 America in forming lines of slackwater navigation, and has been 

 found to stand remarkably well. The dam just alluded to, at the 

 Fairmount Waterworks, withstood a great flood which occurred 

 at the breaking of the ice, on the 21st February, 1822, without 

 sustaining the smallest injury. On that occasion the water of 

 the Schuylkill flowed over the top of the dam in a solid body no 

 less than eight feet eleven inches in depth. As the erection of 

 the dam impeded the navigation of the river, the Water Com- 

 pany had to compensate the Schuylkill Navigation Company by 

 forming a canal, for the passage of their coal barges. This canal 

 is about 900 feet in length. It has two locks of six feet lift 

 each, and one guard lock at the upper extremity. 



The water is admitted into the mill-race by three archways 

 which have a water-way sixty-eight feet in breadth, and, when 

 the river is in its ordinary state, admit a body of water six feet 

 in depth. These archways can be shut by means of gates, and 

 the whole of the water can be drawn off from the mill-race, if 

 required, by opening a sluice communicating with the part of the 

 river below the dam. The mill-race, which is excavated in solid 

 rock, was a most laborious and expensive work. It is 419 feet in 

 length, and 140 feet in breadth ; its depth varies from sixteen to 

 sixty feet. 



The water flows to the wheel-houses, which have been built of 

 a sufficient size to admit of eight wheels and eight force-pumps 

 being employed to raise the water. In 1837 only six of the 

 wheels and six force-pumps had been put up. The average daily 

 quantity of water raised by each pump during the last year was 

 530,000 gallons, and the whole quantity of water distributed from 

 the reservoirs per day, to 19,678 householders, was 3,122,664 

 gallons. It has been calculated that thirty gallons ^of water, 

 acting on the wheel, raised one gallon irto the reservoir. 



The water-wheels vary from fifteen to sixteen feet in diameter. 

 They are fifteen feet in breadth, and make thirteen revolutions 

 per minute. The spokes, rims and buckets are formed of wood, 

 but they revolve on cast-iron axles, weighing five tons each. The 

 working of the wheels is impeded during spring tides, by the 

 water rising upon them ; but it has been found that their motion 

 is not materially affected until the back-water rises about sixteen 



