MECHANICS OF LIQUIDS 



77 



are the centers of distributing systems that cover hundreds 

 of miles of territory, and give electrical service to many 

 towns. 



77. Measurement of Flowing Water. Oftentimes, as when 

 water is sold to a corporation or city, it is necessary to know the 

 quantity of water coming 

 down a stream. To mea- 

 sure this a device called a 

 weir (Fig. 49) is constructed 

 at the sides of the stream so 

 as to form either a rec- 

 tangular or angular opening 

 through which the water 

 flows. Where a large quanti- 

 ty of water is measured the 

 opening is usually shaped in 

 this way I ] ; where 



FIG. 



49. Measuring Water 

 Weir. 



with 



the quantity is small the 



opening is V-shaped, as the flow of water may then be measured 



with greater accuracy. 



The volume of the flow is measured by ascertaining the height 

 of the water above the bottom of the notch. To do this a peg is 

 driven into the bed of the stream as at E in Fig. 49. The top of 

 this peg is exactly the same height as the bottom of the notch. 

 A measuring scale inserted in the water as shown in the illustration 

 then enables the exact height of the flow over the weir to be measured. 

 The formula for determining the volume of flow is : 



Q = X B X#l/2 gH 

 lo 



Where Q = cubic feet passing over the notch per second, K = .59, 

 which is a constant, B is the breadth of the water in the notch, H 

 is the height of the water in the notch, g = 32.2 (force of 

 gravity). 



The energy stored in the moving water is equal to the number 



