256 The Data on which we have to depend 



pond (luring nine months, the same amount must be added to 

 the evaporation, and, therefore, nothing is gained. 



It is a singular fact that all our hopes of deriving an ade- 

 quate supi)]y of water from Yan Yean, at this moment, de- 

 pend on Mr. Hodgkinson's estimate of the evaporation derived 

 from the pond, and it must be regarded as still more singular 

 that his own data, on Avhich lie rehcs to prove the correctness 

 of his estimate, have furnislicd the best proof of its fallacy, 

 by clearly sliowing that the pond is supphed from springs. 



In calculating the evaporation of the other nine months, 

 Mr. Hodgkinson has recourse to English data, and computes 

 the amount by " correcting these for the average differences 

 of temperature for the various months of the year," and by 

 " applying a slight additional correction for the frequent oc- 

 €urrence of di^y winds." 



He does not say whose tables he has employed tor tins 

 purpose, but, in order to illustrate the principle upon which 

 he proceeds, I have added to the foregoing tables the evapo- 

 ration from the surface of water at Manchester,^ according to 

 Dr. Dalton's experiments, and also the evaporation from the 

 surface of water in this colony, deduced from the English 

 evaporation by allowing the same proportion to the same mean 

 temperature in both countries. 



For our three summer months I have adopted Mr. Hodg- 

 kinson's estimate of eight inches, and the result, as may be 

 seen by reference to the tables, gives five feet 1 1 -85 inches. 



Thus it is seen that the corrections for temperature alone, 

 give 5-25 inches more than Mr. Hodgkinson allows, after 

 having made all the additional corrections that arc necessary 

 for our very dry atmosphere, and the more intense action 

 of our very dry north winds. , -, , . , ^ 



But it is not necessary to adopt Mr. Hodgkinson s estimate 

 in order to get eight inches of evaporation for each of our 

 three summer months. 



Dr. Dalton's tables give 6-49 inches as the evaporation lor 

 June in Manchester, and they also point out the method tor 

 correcting the evaporation for temperature and dryness, by a 



simiile formula. 



Now, Dr. Davey has shown that the temperature ot our 

 three hottest months is 10^ higher than the temperature ot 

 the three corresponding months in London, accor(hng to the 

 tables of the Royal Society. Thus, our increased tempera- 

 ture alone, without reference to dryness, would give 

 inches as the evaporation from the surface of water deduced 



