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 w^atershed. 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 amoimt 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 Grreat 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 calcvilated 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 

 ai'ea, 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 



