256 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



we have Jc V 2 as an expression for the double 

 head actually produced by the gage. The 



2 

 ratio of Jc to - measures the efficiency of the 



9 



instrument in realizing the theoretic head. 



the 



ure of efficiency is 



2 32.16 1 0.996 



32.28 A 2 ' 2g 32.28 A 2 ~~ A 2 



The values of the measure have been com- 

 puted and are given in Table 83. The deep- 

 water and mid-depth ratings being assumed to 

 represent normal conditions, the mean of the 

 corresponding efficiencies, namely, 0.62, may 

 stand in a general way for the fraction of the 

 theoretic head which is realized by this particu- 

 lar type of the Pitot-Darcy gage. 



As the relation between velocity and 

 observed head is that between agent and effect, 

 it is evident that values of the efficiency ratio 

 rather than those of the constant A should be 

 compared in any attempt to explain the phe- 

 nomena of variation. Restating from this 

 viewpoint the results of comparative ratings, 

 we have: The response of the head to changes 

 of velocity is lessened when the receiver of the 

 gage is brought near the free water surface and 

 is increased when the receiver is brought near 



a rigid part of the stream's perimeter. It may 

 be surmised that the differences in head are 

 connected with the facility with which the 

 flow lines of the water are diverted in passing 

 around the instrument, regarded as an obstruc- 

 tion. In midcurrent the diversion is resisted 



Water surface 



Bottom or side 



FIGURE 87. Graphic table for interpolating values of A, in V= 

 A -jHHi, for observations made with gage 3b in different parts of a 

 stream. 



by the inertia of surrounding water. Near 

 the surface the resistance of water is partly 

 replaced by resistance of more mobile air. 

 Near the conduit the resistance of water is 

 partly replaced by the resistance of an immobile 

 solid. 



