For our Water Supply, 



257 



from English tables^ the rate of evaporation being doubled 

 with every increase of 20^ of Fahr. 



But Dr. Davey has also shown that the mean dew-point of 

 our hottest months is 50*, thus showing a dryness of 19^^ or 

 in the proportion of two and one-fourth to one compared 

 with London. 



Now, according to the formulce of Dr. Dalton, and Dr. 

 Ure, 9*73 inches corrected for our dryness, would give 16*22 

 inches for each month, or 48*66 for three months. 



The above table shows that the English evaporation for the 

 other nine months, corrected for temperature alone, is 47*85 

 inches. 



As there are no correct data to show our relative dryness 

 for these months, this correction must be omitted, but, even 

 without this, the rate of evaporation deduced from English 

 tables, in the manner described by Mr. Hodgkinson, amounts 

 to eight feet 0*5 1 inches. 



Thus, if we make some additional corrections for our greater 

 dryness for the nine months, and for the more " intense action" 

 of our dry winds, I do not see how Mr. Hodgkinson can 

 escape from the conclusion, even according to his own method 

 of calculation, that the evaporation from the surface of water 

 in this colony is little short of nine feet. 



In my former paper I stated, on the authority of the Year 

 Book of Facts for 1854, that Mr. Glaisher had estimated the 

 evaporation at Greenwich at five feet ; I have now ascertained 

 from his Hygrometric Tables, that his estimate is four feet 

 two inches, so that Mr. Hodgklnson's estimate of five feet 6*6 

 inches for Melbourne, is very little more that Mr. Glaisher's 

 for Greenwich, and the Greenwich evaporation when corrected 

 for our higher temperature, would give six feet three inches 

 without any corrections for dryness and winds. 



But why does he resort at all to English data in order to 

 deduce our evaporation in a troublesome and unsatisfactory 

 manner ? Had Dr. Dalton any peculiar method of determin- 

 ing the evaporation at Manchester different from that adopted 

 by Dr. Davey in Melbourne ? If it Is correct to deduce our 

 evaporation from Dr. Dalton's evaporation for the nine months, 

 why is it incorrect to depend on Dr. Davey's evaporation for 

 the three months ? 



Both these gentlemen have adopted precisely the same 

 method of experimenting in determining their respective rates 

 of evaporation. 



Dr. Davey's experiments, which were conducted daily 

 during the period referred to, are in every respect similar to 



