124 Failure of the Yan Yean Reservoir. 
reservoir from this source, and in the same proportion twenty- 
two inches represent the rainfall for the six winter months. 
Having ascertained the rainfall of the Plenty basin it 
would be of great importance to determine the whole amount 
of the watershed. The only certain method of obtaining 
this result would be to take accurate measurements of all the 
tributaries, at least once in each month, and to make a careful 
survey of the floods that may occur during the year. 
In the absence of such measurements it becomes important 
to estimate the amount from data that are recognised in 
England, making due allowance for the difference in the mean 
temperature, and the physical peculiarities of the drainage 
area. 
If we could ascertain the amount of rain in any district, 
and the proportion of the rain that is evaporated from the 
surface of the ground, the difference would exactly equal the 
contents of the rivers. ; 
On this principle, the late Dr. Thomson, the Professor of 
Chemistry in the University of Glasgow, estimated the 
watershed of Great Britain at four inches of the rainfall. 
From the meteorological tables he calculated the rain, 
including four inches of dew at thirty-six inches, and from 
experiments and observations he calculated the amount of 
evaporation from the ground at thirty-two inches. He 
therefore computed the watershed at four inches, or one-ninth 
part of the rain. 
Although Dr. Thomson considered his estimate of four 
inches too high, from a calculation which he made of the 
the contents of the river Clyde, compared with its drainage 
area, yet it differs so greatly from other estimates, which are 
as high as eleven and thirteen inches, that the only inference 
we can draw is that the whole subject is still enveloped in so 
much obscurity and uncertainty, that no correct practical 
results can be obtained by this method. 
It would, therefore, be altogether a visionary speculation, 
to make the water supply of a large city depend upon the 
correctness of either of the higher estimates. It would be 
unworthy of modern engineering science, and could only 
lead to failure and disappointment. 
The most correct view of the subject is probably that 
entertained by Dr. Prout, who thinks that the truth lies 
somewhere between the extremes, and I therefore feel dis- 
posed to determine the watershed of England, in accordance 
with the experiments of Messrs. Hoyle and Dalton, who 
