258 The Data on which we have to depend 
those which are everywhere else depended on for ascertaining 
the rate of evaporation, and they were conducted with a degree 
of care and minute accuracy which it would be difficult to 
exceed. He carefully measured, in a graduated vessel, each 
portion of water that was exposed to evaporation, and thus 
every drop that was evaporated was accurately registered. 
Those gentlemen who question the accuracy of his results 
ought to point out in what manner his experiments differ from 
Dr. Dalton’s or Mr. Charnock’s, and how it is that his are 
fallacious while they place implicit confidence in theirs. 
The method commonly adopted for the purpose of throwing 
doubts on the accuracy of Dr. Davey’s results is to compare 
his scientific experiments with observations on ponds and 
waterholes. But enough, I trust, has already been said to 
show that a more fallacious test could not be applied. 
But, if this question must be decided by observations on 
ponds, I have mentioned other observations which give nearly 
double the amount of Mr. Hodgkinson’s estimate, and I do 
not see in what manner he can dispose of these. And I my- 
self measured, with the greatest care, the evaporation from 
the surface of a pond in the month of February, and found 
that there was a loss of exactly eleven inches in twenty-eight 
days. Now, can Mr. Hodgkinson point out any source of 
error in this experiment? unless it is that I omitted to add 
anything for rain, or dew, or lateral supply, for all of which 
he has made a very liberal allowance in his experiment, but 
this would have added to, not diminished the rate of evapora- 
tion. 
The only scientific objection that has been urged against 
Dr. Davey’s estimate being applied to the Yan Yean Reser- 
voir is the great extent of surface. It is thought that the air 
will become so saturated with vapour that the rate of evapora- 
tion will be very much diminished. 
There can be no doubt that inthe case of the ocean this 
objection would have considerable weight, though, even there, 
extended observations show that the air is very rarely near 
the point of saturation; but with regard to the Yan Yean 
Reservoir, I feel quite certain that the effect which extent of 
surface would have in retarding evaporation has been greatly 
exaggerated, 
Being surrounded by an amphitheatre of hills, it may, to a 
certain extent, be protected from strong winds; but, on the 
other hand, this physical conformation will render it more 
liable, in calm weather, to atmospheric currents resulting from 
the unequal effects of solar heat on the surface soil of the 
Seca toe yt 
