168 HYDBAULICS AND ITS APPLICATIONS 



length b and under a head H. 1 This determines H, and therefore the 

 total rise in level when the height of dam is given. Thus if h = mean 

 depth of water before the introduction of the dam, and if h d = height of 

 dam, 



we have Q = | C b V~^~g H* 



... H={ !g_ 



\CbV2g 



Eise in surface level I _ 7 , . _ , 

 produced by the dam f : 



The same reasoning applies to the case of a submerged weir thrown 

 across a stream, or to the rise in level produced by the erection of bridge 

 piers in the stream (Art. 91). 



ART. 59. USE OF THE WEIR AS A WATER MEASURING APPLIANCE. 



The standard sharp-edged weir having a free discharge, or, for small 

 quantities, the right-angled triangular notch, are the only types for which 

 the coefficients have been determined with sufficient accuracy to admit of 

 use for accurate measurement of flow without previous calibration. 



For accurate measurement the following are essentials : 



1. Sharp-edged weir sill, fixed so as to be incapable of vibration, having 

 its face vertical and perpendicular to the direction of the stream, and, if 

 rectangular, having its sill horizontal. 



2. Clear discharge into air, no adherence of vein to weir face. 



3. Weir long in proportion to its depth, i.e., b > 3 H. 



4. H small in comparison with the depth of the approach channel, and 

 sectional area of vein (b H) not greater than J that of this channel in a 

 weir with end contractions, or not greater than J its area with a suppressed 

 weir. 



5. Suitable channel of approach. This should be as long and of as 

 uniform section as possible so as to allow of the motion becoming steady 

 before reaching the weir. The length should, if possible, exceed 30 H, this 

 ratio being increased where the length of weir is largely in excess of 3 H. 

 In Bazin's experiments the length of supply channel was 49'2 feet with a 

 maximum head of 1*97 feet and a maximum weir length of 6*56 feet, giving 

 a length = 25 H. In Messrs. Fteley and Stearns' experiments the length 



1 This velocity may be determined by current meter or float observations. See Arts. 98 100. 



