360 



NEW YORK STATE MUSEUM 



Duncan Companj^ who has used the Francis formula for the 

 Merrimac daru, namely: 



Q=3.012 L H 153 ; (36) 



in which — 



L = length of dam = 794 feet ; 



H == depth on crest of dam, in feet. 



This formula has been used in all cases, whether flashboards 

 are on or off. Mr Bloss states that his reason for using this 

 formula is that there was a litigation at Mechanicville in which 

 the quantity of water flowing over the dam became an important 

 element. He therefore used the Francis formula for the Merri- 

 mac dam because the courts were familiar with this formula, 

 whereas, had he continued to use the East Indian engineers' 

 formula, the courts would not be familiar with it and might not 

 accept it. The difference between the two formulas is not very 

 great. At 4 feet depth it is about one cubic foot per second 

 per foot of crest, which would make at that depth 794 cubic feet 

 per second for the entire dam. Probably the greatest oversight 

 in this computation is the use of the formula for the Merrimac 

 dam when the flashboards are on. At 4 feet depth the variation 

 between the formula for the Merrimac dam and Francis formula 

 for a sharp-crested weir is about 13 cubic feet per second per 

 linear foot of dam, and even at 2 feet depth on crest the varia- 

 tion is over 6 cubic feet per second per foot of dam. It is con- 

 cluded, therefore, that the computations from 1899 to date are 

 somewhat less reliable than those of the previous years. The 

 following cut shows a section of the Mechanicville dam. 



U — About 8'— J 



Fig. 26 Section <>f Mechanicville dam. 



