256 The Data on which we have to depend 
pond during 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 supply 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 which he relies to prove the correctness 
of his estimate, have furnished the best proof of its fallacy, 
by clearly showing that the pond is supplied 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- 
currence of dry winds.” 
He does not say whose tables he has employed for this 
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 af Manchester, according to 
Dr. Dalton’s experiments, and also the evaporation from the 
surface of water in this colony, dedtfced 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, andthe result, as may be 
seen by reference to the tables, gives five feet 11:85 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 are 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 for 
June in Manchester, and they also point out the method for 
correcting the evaporation for temperature and dryness, by a 
simple formula. 
Now, Dr. Davey has shown that the temperature of our 
three hottest months is 10° higher than the temperature of 
the three corresponding months in London, according to the 
tables of the Royal Society. Thus, our increased tempera- 
ture alone, without reference to dryness, would give 9°73 
inches as the evaporation from the surface of water deduced 
