130 - Failure of the Yan Yean’ Reservoir. 
But how could such a flood be secured for the reservoir ? 
The same amount of water would take twelve days to pass 
through the aqueduct at one mile an hour, which is the high- 
est velocity that it would be either safe or prudent to allow. 
A higher than this, in such a winding canal cut out of the 
clay-slate, would convert the water into mud, and break up 
the sides, and wash away the artificial banks; and for the sake 
of such a flood, which may possibly occur once in ten years, 
would it be reasonable to spend £30,000 in enlarging the aque- 
duct to four times its present size? and without suitable 
embankments raised at an enormous cost, to dam up such a 
flood, one half would not enter the aqueduct at all. 
As twelve inches of rain fell during the month, instead 
of four, we may safely conclude that this flood resulted from 
a rainfall of at least six inches, which would give thirteen 
feet in the reservoir, so that out of thirteen feet of rainfall 
in one of the heaviest floods on record, only three feet reach 
the rivers, or less than one-fourth. 
The only other source of supply to be noticed is dew. In 
England from four to five inches have been computed as the 
amount of dew deposited on the ground, but I am not aware 
of any experiments to show the amount deposited on water. 
I feel persuaded that the atmosphere is generally so dry here, 
that the amount of dew must be very small; and unless in 
the case of very shallow pools and lakes, there can be very 
little deposited on water. A depth of two or three feet will, 
in a great measure, prevent the formation of dew, because as 
the upper particles become cooled they at the same time 
descend, from their increased density, to make way for the 
warmer and lighter particles underneath, and until the whole 
depth of water has attained a considerably lower temperature 
than the atmosphere with which it is in contact, no deposition 
of dew can take place. 
As it is possible that there may be some dew, when there 
is very little water in the reservoir, I shall on this account 
allow an increase of two inches for the whole surface, which 
is equal to sixteen and one-half days supply for the city. 
The whole amount will stand thus :— 
From the River Plenty - - - 5ft. 6in. 
Floods in ditto - - = - ve 
Increase from winter rain - - - 
Rainfall in reservoir . 4 “5 a 
0 
i lalbatoas b 
3. 6 (OO 
Drainage area of ditto - - - 1 2 
Dew - - - - - =i OL a2, 
ll. 63 
Total amount 
