For our Water Supply. 



251 



And I feel persuaded that Mr, Hodgklnson 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 



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 EJnglish data. Now, these data represent the amount 

 of water evaporated, as determined by actual measurement, 

 Avithout any reference to dew, the condensation and evapora- 

 tion of which on the surface of the evaporating vessels arc 

 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. Griaisher'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 Avill 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 

 suflScient 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, Avhich is five feet 6 '6 inches, to Dr. Davev's 



