METROPOLITAN WATER SUPPLY. 123 



annum ; but of course every basin must be judged by itself, 

 taking into account rainfall, the nature of the soil, temperature, 

 and many other considerations. Observations of actual discharge 

 for a year or two, with a comparison of the rainfall of the years 

 observed, with the average registered rainfall, &c, are the 

 only guides in estimating the probable yield of any watershed. 



" There has never been any scarcity of water in the Tan Yean 

 reservoir. I suppose the annual consumption may be taken at 

 3,500 million gallons, or somewhere about 51 million gallons from 

 the square mile ; so that a great deal more than half the water 

 may be supposed to run to waste, that is, sent down the river 

 and, shut out from the feeding channel of the Yan Yean reservoir. 



" Roughly speaking, I should think that you would require from 

 80 to 100 square miles of drainage area to supply 200,000 people, 

 supposing the rainfall to be 35 inches and the country moderately 

 well suited for discharging a fair proportion of it." 



By this letter I derive more and more confidence that the 

 watershed and supply from my proposed gathering country is 

 equal to the gathering ground for the fine Yan Yean supply, 

 added to which we have to remember that against the average 

 rainfall in Victoria of 35 inches, we have, even at our " South 

 Head " at Sydney itself, an average rainfall for the past sixteen 

 years of 52*375 inches, whilst on the sources and country of my 

 supply I doubt not that the average rainfall exceeds 60 inches. 

 Therefore, with my knowledge of the available watershed, as 

 proved by my survey as far as Eeeve's Creek, south, I could only 

 consider it to be quite puerile to question the superabundant 

 water supply at our command for my high gathering and gravi- 

 tation scheme for any amount of population that may hereafter 

 grow up in Sydney and its suburbs. 



"With these general remarks to be attached to my late paper, I 

 now resign my water supply scheme into your hands. 



