For our Water Supply. 241 
well-drained soil in Yorkshire, Mr. Howard’s apply to the 
average surface of England; and it is interesting to remark 
that the observations of these gentlemen corroborate the 
previous observations of the late Dr. Thomson, of Glasgow, 
who estimated four inches ag the watershed of Great Britain, 
from observations and measurements of the Clyde. 
It is to be regretted that Mr. Hodgkinson has not clearly 
stated on which authority he has based his estimate of five 
inches, or the precise method by which he has arrived at this 
very important conclusion. 
He describes Mr. Charnock’s observations as the most ex- 
tensive and minutely accurate ever made in Britain, but they 
apply only to the Eastern Counties of England, where the rain- 
fall averages twenty-four inches. They also apply exclusively 
pervious well drained soil, and are, therefore, not applicable 
to impervious undrained lands, which receive and evaporate 
a large portion of the watershed from lands that are pervious 
and well drained. Mr. Charnock’s estimate is, therefore, too 
high for the average surface of England, and, with a mean 
annual rainfall of thirty-six inches, would give seven inches in- 
stead of five anda half inches, which is Mr. Howard’s estimate. 
It would be clearly wrong, therefore, to assume Mr. Char- 
nock’s proportion of available rain, for pervious and well- 
drained land in Yorkshire to determine the watershed of the 
Upper Plenty, where there are many thousand acres of im- 
pervious and undrained lands; and, in computing the pro- 
portion of the available rain for the average surface of 
England, Mr. Howard has no doubt made the necessary de- 
duction from Mr. Charnock’s estimate. Hence, while the 
estimate of the latter is one-fifth of the rain, that of the 
former is only one-sixth, and Dr. Thomson’s estimate for 
Great Britain, excluding dew, is one-eighth. 
The mean rainfall for Melbourne, for a period of six years, 
has been found to be 30°85 inches, and Mr. Hodgkinson 
seems to have adopted this proportion of rain for the Upper 
Plenty, as he regards the rainfall and dew taken together, as 
equivalent to thirty-six inches. 
Without any correction, therefore, for temperature or dry- 
ness of the atmosphere, Mr. Charnock’s proportion of the 
available rain would give 6:11 inches for the Upper Plenty, 
and Mr. Howard’s 4°73 inches. 
On what principle, then, does Mr. Hodgkinson adopt 
five inches to represent the watershed of the Plenty basin? 
He says, “I believe, therefore, that the proportionate 
amount of the rainfall available in the Upper Plenty district 
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