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BULLETIN 906, TJ. S. DEPARTMENT OF AGRICULTURE. 



miners-inch boxes, automatic registers, and other mechanical record- 

 ing devices. 



Venturi meters. 



These make use of the Venturi principle by forcing water through 

 a small throat in a pipe, thereby converting most of the static into 

 velocity head. By keeping a continuous record of the normal static 

 pressure and also the pressure at the throat the flow of water at any 



time can be com- 

 puted for a meter of 

 a given size. Meters 

 of this type are in- 

 stalled on main pipe 

 lines when it is de- 

 sired to keep a con- 

 tinuous record of the 

 total quantity of 

 water delivered to 

 any system. They 

 might also be in- 

 stalled advanta- 

 geously on lateral 

 pipe lines were it not 

 for their first cost, 

 since they have no 

 moving parts liable 

 to be obstructed or 

 clogged by sand, 

 leaves, or other ma- 

 terial carried by the 

 water. 



Weirs. 



The weir is the 

 most commonly used 

 device for measuring water in open ditches and canals. It is more 

 difficult to adapt it to pipe systems, and when this has been attempted 

 due consideration has seldom been given to the effects produced on 

 the accuracy of the measurement by seemingly trivial changes from 

 the standard specifications. One of the essential requirements of 

 the weir is that the water shall approach the weir notch at a slow 

 velocity and unaffected by eddies or cross-currents. These condi- 

 tions are seldom fulfilled when water issues from a pipe directly 

 above the weir. 



Fig. 10. — Twelve-inch overflow stand with 6-inch pipe inside. 



