HYDROLOGY OF NEW YORK 



133 



are extremely unsatisfactory. In one case of a flat-crested weir, 

 the flow at a given depth is only 75 per cent of what it is over a 

 sharp-crested weir. Variations of from 5 per cent to 20 per cent 

 are common, as may be easily observed by examining the tables in 

 the paper on the flow of water over dams. 1 



In view of the importance which gagings are now shown to bear 

 in estimating the value of a stream for water power or city 

 water supply, in future every statement of stream flow should 

 be accompanied by a concise statement of the method of gaging 

 used, thus permitting hydrologists to judge of the general relia- 

 bility of the method. Had this been done in the past, some of 

 the uncertainty which now attaches to many gaging records 

 would undoubtedly be removed. 



Genesee and Hudson gagings reduced to sharp-crested weir 

 measurements. The writer has shown in another place that Gen- 

 esee river gagings have been reduced to sharp-crested weir meas- 

 urements. As to the Hudson gagings, pi. CXXVII in the Keport 

 to the United States Board of Engineers on Deep Waterways, may 

 be cited. This plate is a comparison of the discharge over weirs 

 by different formulas, and it appears from it that Mullins's for- 

 mula for a flat-crested weir, which has been used for the Upper 

 Hudson gagings, at a depth of 4 feet gives results less than Fran- 

 cis's formula for a sharp-crested weir by about 10 per cent. How- 

 ever, in order to simplify the computation and to avoid velocity 

 of approach, the width of the crest was taken at 5 feet. Again, 

 the crest at Mechanicville is not flat, but is slightly sloping back- 

 ward. The sloping front probably affects the flow to increase it 

 somewhat. There are also flashboards used during low water, 

 which are properly computed by Francis's formula for a sharp- 

 crested weir. These several elements undoubtedly make the 

 problem somewhat complicated, but taking everything into 

 account it is probable that the results as computed are not far 

 from right. They may, however, be in error as much as 2 inches 

 per year. 2 



: On the Flow of Waters over Darns; loc. cit. 



-See the diagrams of Hudson and Genesee rivers on this point 



