258 



The Data on which we have to depend 



those which are everywhere else depended on for ascertaining 

 the rate of evaporation, and they Avere 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. 



Tlaose 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 arc 

 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 olE* February, and found 

 that there Avas a loss of exactly eleven inches in twenty-eight 

 days. Now, can Mr. Hodgkinson point out any source of 

 error in this experiment ? imless 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 in the case of the ocean this 

 objection would have considerable Aveight, though, even there, 

 extended observations shoAV 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 Avould 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 Avinds ; but, on the 

 other hand, this physical conformation Avill render it more 

 liable, in calm Aveather, to atmospheric currents resulting from 

 the unequal effects of solar heat on the surface soil of the 



