For our Water Supply. 251 
And I feel persuaded that Mr. Hodgkinson could not have 
reflected sufficiently on the general principles which regulate 
the production of dew, otherwise he would certainly have 
omitted it altogether from his calculations, and his extreme 
confidence in a very abundant supply for 191,500, would have 
been considerably diminished. 
And there can be no objection to my deducting ten inches 
from Mr. Hodgkinson’s estimate, or an equivalent to supply 
22,727. 
But there is another view of the subject equally fatal to 
the assumption of ten inches of dew. Mr. Hodgkinson has 
calculated the evaporation from the surface of the reservoir 
from English data. Now, these data represent the amount 
of water evaporated, as determined by actual measurement, 
without any reference to dew, the condensation and evapora- 
tion of which on the surface of the evaporating vessels are 
regarded as balancing each other. ‘Therefore, if ten inches of 
dew are assumed to be condensed on the surface of the 
reservoir, this amount must be added to the rate of evaporation 
deduced from English data. But Mr. Hodgkinson has not 
done this, he has allowed one inch for the three summer 
months in estimating the evaporation of the pond, but it does 
not appear that he has added nine inches for the other nine 
months. 
If he has done this, his estimate of the evaporation, ex- 
cluding dew, would be four feet 9°6 inches for twelve months, 
which it will surely be admitted is a very small allowance for 
this country, when Dr. Dalton’s estimate for Manchester is 
three feet eight inches, and Mr. Glaisher’s estimate for 
Greenwich is four feet two inches. 
If Mr. Hodgkinson, therefore, insists on retaining ten 
inches of dew in his estimate, he cannot object to add nine 
inches to his evaporation, which will thus amount to six feet 
3°6 inches; but in this case the dew goes for nothing. 
5. I have thus far endeavoured to show that very large 
deductions must be made from Mr. Hodgkinson’s estimate, 
ere we arrive at the amount that will be available for the 
supply of the city; and his estimate for 191,500 has been re- _ 
duced by an amount that would’ supply 109,337, leaving still 
sufficient for 82,163. 
I now proceed to consider what dependence is to be 
placed on the amount gained by Mr. Hodgkinson, from 
. prefering his own estimate of the evaporation from the surface 
of the reservoir, which is five feet 6°6 inches, to Dr. Davey’s 
which is nine feet. 
