For our jVater 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 as 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 E ngland, where the rain- 

 fall averages twenty-four inches. They also apply exclusively 

 pervious well drained soil, and are, therefore, not applicable 

 to impervious undralned 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 and a 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 undralned 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 



E E 



