For our Water Supply. 253 
where all are equally accessible, how some should be emptied 
by cattle, while others, apparently, lose very little-water. 
And this objection cannot apply to my observations with 
reference to the Deep Creek, as the land is enclosed for 
cultivation. 
The water of these ponds is lost, therefore, either by 
evaporation, or absorption. Hither admission would be alike 
fatal to the prospects of the Yan Yean Reservoir. 
If so much water can be absorbed through the slate strata 
which form the bed of the Deep Creek, what reasonable 
grounds have we to expect that the same amount of absorption 
will not take place through the slate strata that form the bed 
of the reservoir ? 
It may be noticed that some settlers have great confidence 
in the Yan Yean scheme from observing that small artificial 
water-holes are often permanent in the summer months. 
If my reasoning is correct with regard to the effects of 
evaporation in this country, we may assume that the evapora- 
tion from the surface of water is nine feet, and that one-ninth 
of the rain may be relied on as the watershed. 
The extent of drainage area necessary to give a permanent 
supply of water to any pond can, therefore, be easily 
determined. 
With a rainfall of thirty-six inches, the ratio of the drainage 
area to the surface of the pond must be greater than eighteen 
to one, in order to secure a permanent supply. 
The ratio of the Plenty basin to the surface of the reservoir 
is about twenty-seven to one, but more than one-third of the 
watershed is not available for the reservoir, a large amount 
being lost in the swamps, and it being necessary to leave a 
certain proportion to maintain the flow in the river. 
Thus the ratio is practically reduced to eighteen to one, 
and there is, therefore, no more than sufficient to cover the 
evaporation. 
Reservoirs in England seldom exceed fifty acres, and they 
are generally much smaller, hence the loss from evaporation 
is very trifling, and the area of surface drained very large in 
proportion. 
The reservoir which supplies New York is 400 acres, with 
a depth of forty feet, and an unlimited command of water, 
the loss from evaporation is, consequently, not equal to one- 
third of that which will be sustained at Yan Yean. 
Had the Yan Yean Reservoir not exceeded 400 acres there 
would have been a saving of water equivalent to supply 
188,500, at forty gallons per head, per day, with a depth of 
fifteen feet eight inches, instead of four feet four inches. 
