HYDROLOGY OF NEW YORK 



141 



Professor Coffin's results were obtained by observing the varia- 

 tions of weight of a dish of water of the same size as the rain 

 gage, with which the evaporation determinations are compared. 

 This method would probably give results considerably in excess of 

 the truth. Moreover, a single year is too short a period for safe 

 results. 



Croton Water Department evaporation records. Table No. 29 

 presents the results of evaporation for certain indicated months 

 (1) for the years 1867-1870, at Boyd's Corners storage reservoir, 

 Putnam county, as determined by J. J. B. Croes, 1 from a wooden 

 tank sunk in the earth; (2), for certain indicated months, 1864- 

 1869, inclusive, from a wooden tank sunk in the earth at the 

 receiving reservoir in the City of New York; (3), from a wooden 

 tank in a batteau at the receiving reservoir in New York, and 

 (1), from a tin box in a batteau at the receiving reservoir in New 

 York. 



The foregoing evaporation experiments are referred to in a 

 paper by Mr Croes on the liow of the West Branch of Croton river. 

 The gage used was a tight wooden tank 4x4x3 feet, sunk in the 

 earth in an exposed situation and rilled with water. As indi- 

 cated, the mean evaporation at Boyd's Corners for the 

 indicated months was 24.47 inches, while at a similar 

 tank at the reservoir in New York city it was 34.06 

 inches. Mr Croes attributes the difference in these results 

 to the different methods of observation and measurement, 

 and states that the Boyd's Corners observations were made twice 

 a day, and any discrepancies that might have occurred were thus 

 found and corrected at once, while the observations at the reser- 

 voir in New York city were made only once a month, the differ- 

 ence between the reading of the gage on the tank and the ob- 

 served rainfall being taken as evaporation. Mr Croes therefore 

 considers the work done at Boyd's Corners reservoir as more 

 reliable. 



l Trans. Am. Soe. C. E., Vol. Ill, 1874. 



