Failure of the Yan 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 gallons, or 
ninety gallons in all, and this is precisely what is frequently 
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 of 
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 in or- 
der that a little may be saved to allay the dust in our streets 
and thoroughfares. 
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 of 
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 
thirty gallons. 
It is singularly illustrative of the peculiarities of this 
reservoir scheme to glance at the results arrived at by the 
Committee. 
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 
thie 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 
