For our Water Supply, 



243 



is relied on as correct for the average surface of England, 

 and Mr, Charnock's tables, which are alone applicable to 

 previous well-drained lands, give only 6'11 inches as the 

 proportion of available rain for the Upper Plenty District, 

 and 6'11 inches, if duly corrected in the manner described 

 by Mr, Hodgkinson, would give considerably less than four 

 inches. I am at a loss, therefore, to discover by what 

 method he has arrived at his conclusion that five inches of 

 available rain represent the watershed of the Plenty basin. 

 There is nothing in his reasoning to show why he should not 

 rather have adopted four inches, but the reverse. 



If he has adopted Mr. Charnock's proportion of 6 '11 

 inches, then he has allowed I'll inches for all the contingen- 

 cies to w^hich he refers, and he has given no reasons why he 

 should not rather have adopted Mr. Howard's proportion, 

 which gives, without any correction for temperature, only 4 '7 3 

 inches, for the rainfall of the Upper Plenty. And, as it 

 appears to me, Mr. Howard's proportion for England, with 

 adequate correction for difference of climate, is the only safe 

 proportion from which to deduce the watershed of the Plenty 

 basin. 



I have not had an opportunity of correcting either Mr, 

 Charnock's or Mr. Howard's tables of evaporation, for differ- 

 ence of temperature, but I have in the following tables 

 corrected Dr. Dalton's precisely in the manner explained by 

 Mr. Hodgkinson, and Dr. Dalton's estimate of available 

 rain for England, which is 8"41 inches, when thus corrected 

 gives exactly 4*54 inches as the proportion of available rain 

 for our climate, without any correction for our very dry 

 atmosphere, for which half an inch in addition may be very 

 safely allowed. Thus the conclusion is inevitable that the 

 tables of Mr. Charnock, and Mr. Howard, if similarly cor- 

 I'ected would give a still less result. 



Admitting, therefore, that Mr. Hodgkinson is right in 

 assuming Mr. Charnock's proportion of the available rain as 

 applicable to the Upper Plenty district, I do not think that 

 he has advanced any good reasons to show that the differ- 

 ence in the evaporation of the two countries is so small, as 

 to warrant the very small allowance he makes for the differ- 

 ences of climate, in adopting five inches. 



In my former paper I expressed a very decided opinion 

 that no confidence could be placed in theoretical estimates 

 of the watershed of the Plenty basin, deduced from English 

 data, at the same time, as a subject of scientific interest, 

 rather than of any practical value, I assumed Dr. Dalton's 



