54 SMALL WATER SUPPLIES. 



simple, and reliable piece of apparatus. There is 

 considerable difference in the formulae used to calcu- 

 late the flow over V and rectangular notches. In the 

 case of the former the stream, lines, and general 

 shape of the set remain similar to one another under 

 all heads, while in the rectangular notch a change of 

 head produces dissimilar sets. The idea of using a 

 triangular notch for gauging small flows of water is 

 due to Prof. James Thomson, who has proved that 

 the quantity flowing depends upon the |th power of 

 the head, and a coefficient has been deduced from 

 experimental data thus : 



when Q = discharge in cubic feet per second, 

 h = head of water over notch mfeet, 

 c = coefficient of discharge, in this case 

 equal to 2*635, 



Q-<A)f 



The equation is quite simple to use, but the use of 

 logarithms is necessary. The angle of the V should 

 be 90. 



Now the foregoing method of construction would 

 not be suitable to a greater head than about 4 ft., and 

 when the depth would have to be greater a stronger 

 form of construction must be resorted to. This 

 might be the case when a wooden weir is erected 

 across a river, not necessarily for gauging purposes 

 but to dam back the water for a variety of purposes, 

 in which a permanent brick, stone, or concrete weir, 

 which are described in Chapter V, would be too ex- 

 pensive. A few calculations must be gone" into 

 referring to fig. 45. The greatest pressure likely to 

 come on the bottom of the dam will be equal to wh, 

 where h is the height to top water level as shown, 



