254 The Data on which 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 isit 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. Hodgkinson 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 credibility 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 asingle 
experiment on a pond, attended by many circumstances of 
doubt, and not conducted with that minute accuracy of detail 
which could alone command 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 acres, 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. 
In calculating the evaporation from the pond, the 3°6 inches 
might have been omitted altogether. 
If we refer to Dr. Dalton’s table we shall find that, with a 
rainfall of 4°15 inches in July, 4°09 inches are evaporated, 
leaving only 0:06, or one sixty-ninth part, to represent the 
watershed. 
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. 
Thus 3°26 inches must be deducted from the supply of 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. 
