254 



The Data on ivhich we have to depend 



With thirty Inches of rain the proportion would be twenty- 

 one and half to one, and I have no doubt that, in all those cases 

 in which settlers have obtained a permanent supply from arti- 

 ficial water-holes, the ratio of the surface drained to that of the 

 water-holes would be found to correspond to the proportions 

 indicated above. Nor is it difficult to understand how, with a 

 large area and steep slopes, a small pond might be supplied 

 even from the summer rains. 



Thus, according to the evidence of gentlemen perfectly 

 competent to describe what they have frequently observed, 

 the evaporation from the ponds referred to is at least double 

 what Mr. Ilodgkinson observed in his pond. 



I might multiply instances of a very high rate of evapora- 

 tion that has been observed both in this country, and else- 

 where, by gentlemen whose credibihty cannot be doubted, 

 but, at present, I merely allude to the fact forthe purpose of 

 showing that Mr. Hodgkinson is not justified in making so 

 momentous a question as the rate of evaporation at Yan Yean, 

 and the whole water supply of Melbourne, depend on a single 

 experiment on a pond, attended by many circumstances of 

 doubt, and not conducted with that minute accuracy of detail 

 which could alone comiuand the confidence of scientific men, 

 and without the most distant reference to the experiments 

 and observations of others, who have arrived at very different 

 results from his own. 



Mr. Hodgkinson estimates the area of his pond at one and 

 a-half acrest and the area of the surface which it drains at 

 nine acres. The ratio is, therefore, only one to six. 



And he assumes fifteen per cent, of the rainfall for the wa- 

 tershed, which gives 3 '6 Inches for the three hottest months, 

 from a rainfall of four inches. 



For the Plenty basin he has assumed 13-9 per cent, of the 



rain as available. i i o • i 



In calculating the evaporation from the pond, the 3-b inches 



mlo-ht have been omitted altogether. _ 

 If we refer to Dr. Dalton's table we shall find that, with a 



rainfallof 4-15 Inches In July, 4-09 inches are evaporated, 



leaving only 0-06, or one sixty-ninth part, to represent the 



watershed. -,;r nrx i i • i ^ ^ 



Instead of 3*6 Inches, therefore, Mr. Hodgkinson ought to 

 have added only 0'34 inches, or one-third of an inch, as the 

 watershed from the nine acres. . n ^ 



Thus 3-26 inches must be deducted from the supply ot the 

 pond, and, therefore, from the evaporation, and there only 

 remains 20*74 inches of evaporation for our three hottest 

 months, or 6*91 Inches for each month. 



