Failure of the Van Yean Reservoir. 



151 



would allow thirty gallons instead of fifteen, as it covers more 

 than two thousand acres; and as the evaporation from the 

 surface is more than double that of England, I am satisfied 

 that he would on this account allow another thirty gahons, or 

 ninety gallons in all, and this is precisely what is irequently 

 used in New York on the constant service principle. 



The Committee, it appears, hold Mr. Dempsey in high esti- 

 mation as an authority. I am surprised, therefore, that they 

 do not follow him in his liberal views on the water supply ot 

 cities. They only allow thirty gallons, but if his views are 

 correct, this amount will leave no water for the numerous 

 important purposes which he enumerates; so that we shall 

 have carefully to guard against any unnecessary^ waste m or- 

 der that a little may be saved to allay the dust m our streets 

 and thoroughfares. .tip 



When the object is to obtain a very large watershed from- 

 the Plenty basin, they adopt Mr. Dempsey's evaporation 

 tables which give nearly the highest theoretical estimate ot 

 the watershed for the mean temperature of England, or about 

 three times the amount of Dr. Thomson's estimate, who was 

 at least equally well qualified with Mr. Dempsey to prosecute 

 any scientific investigation; but when the object is to make 

 the most of the limited supply at Yan Yean, they forget Mr. 

 Dempsey and his water-tables, and, knowing that New York 

 frequently consumes ninety gallons per head, they tell us that 

 Melbourne, which is nearly in the same latitude and has 

 much more need of a plentiful supply, ought only to have 



thuty_gallons^^ illustrative of the peculiarities of this 

 reservoir scheme to glance at the results arrived at by the 



At thirty gallons per head per day, one foot eleven 

 inches in the reservoir will suffice for the city for twelve 

 months Taking their own estimate of the evaporation at 

 nine feet, it will thus be necessary, in order to store and 

 preserve one foot eleven inches for the city, to put into the 

 reservoir each year ten feet eleven inches, or about six times 

 the amount required. 



Thus for every gallon of water that will be consumed by 

 the citizens for domestic purposes, and for watering the streets, 

 five will be consumed by evaporation at Yan Yean. 



The contents of the river Plenty represent that small 

 fraction of the rain that nature has rescued for the use of 

 man from the powerful influence of evaporation, under an 



