HYDROLOGY OF NEW YORK 



95 



By observing the relation indicated in the foregoing tabula- 

 tions, together with hight of ground water, one may approxi- 

 mately compute the rainfall from the runoff. In the same way 

 the runoff may be approximately computed from the rainfall. 



Map of average rainfall in the State of New York. On plate 

 XCVITI of the Report of the United States Board of Engineers on 

 Deep Waterways, the writer has given the average rainfall at a 

 large number of stations throughout the State of New York. 

 When this map was prepared considerable time was expended in 

 drawing lines of equal rainfall upon it, but so many discrepan- 

 cies appeared that it was finally concluded, for the present, that 

 it should be allowed to stand without such lines. The only way 

 these contours could be drawn with any satisfaction was to omit 

 stations which conflicted too much therewith. This, the writer 

 did not feel justified in doing. The observations are not exten- 

 sive enough to enable one to draw these lines. 



Length of time required to make good a series of rain-fall records. 

 This question is partially answered in the writer's second report 

 on the Upper Hudson Storage Surveys, for 189G, by a short analy- 

 sis of a paper by Alexander R. Binnie, Member of the Institution 

 of Civil Engineers. 1 



One of the important problems worked out by Mr Binnie is an 

 answer to this question : What is the least number of years 

 the continuous record of which, Avhen a mean fall has been deter- 

 mined, will not be materially affected, as far as the value of 

 the mean is concerned, even if the record be extended by a greater 

 number of years of observation? Mr Binnie says : 



Collaterally, inquiry must also be made if the period necessary 

 to determine the true mean fall is in any way affected by the 

 amount of the mean fall of rain ; or if any approximate rule can 

 be applied to all countries, and to differing amounts of mean 

 annual rainfall at different stations. 



To approach the subject without bias, some good records of 

 long periods must be carefully examined, and it must be found, 

 not how near the averages of the shorter periods into which they 



i On Mean or Average Rainfall and the Fluctuations to Which it is Sub- 

 ject, by Alexander R. Binnie, M. Inst. C. E.: Proc. Inst. C. E., Vol. CIX 

 (1892), pp. 89-172. 



