HYDROLOGY OF NEW YORK 139 



accepting it for one month as true, because rainfall or snowfall 

 at the end of the month can be easily carried over to the next. 

 This is also true sometimes for two months, but for the present 

 it seems quite doubtful that other than in exceedingly rare cases 

 would negative evaporation occur for three consecutive months. 

 Its occurrence for six consecutive months, or for the entire storage 

 * period, is believed to be impossible. It may, however, be again 

 pointed out that its occurrence renders an attempt at monthly 

 diagrams showing the relation between rainfall and runoff absurd. 



Assuming that the foregoing propositions are reasonably true, 

 it follows that the frequency of the occurrence of negative 

 evaporation in gaging records may be in some degree a criterion 

 as to their accuracy. The writer, however, does not wish to urge 

 this very strongly, but merely to point it out as a possibility. 

 The writer has no desire to be insistent on this point. There is 

 very little on the subject of negative evaporation in engineering 

 literature, and the writer will be glad to have the observations 

 and conclusions of others. 



In a report on the flow of the river Thames, by A. R. Binnie, 

 Chief Engineer of the London County Council, 1 the matter of nega- 

 tive evaporation is elaborately discussed, and in order to obtain 

 all the information possible about it Mr Binnie applied to 

 George J. Symons, F. R. S., to assist him in arriving at some 

 approximate idea on the subject. Mr Symons submitted an exceed- 

 ingly lucid and conclusive report. Eleven distinct cases of nega- 

 tive evaporation were submitted to him for study and comment. 

 In regard to these he arrived at the following conclusions : 



1) Under normal conditions a fall of rain will increase the flow 

 at Teddington weir on the second day after it falls. 



2) Under normal conditions the water running off from any 

 given fall of rain will all reach Teddington weir before the tenth 

 subsequent day. 



3) In the winter an interval of two months, or in extreme 

 cases even more, may elapse between the precipitation of moisture 

 from the clouds and its flow over Teddington weir. 



Report on the Flow of the River Thames, by A. R. Binnie. Publication 

 of the London County Council, dated November 1, 1892. 



