58 
Arterial Drainage 
Fig. 11. — Weir, with Notch-hoard and Peg for measuring the Quantity 
of Water flowing down a Stream. 
stream be dammed up by the weir sufficiently to reduce it nearly 
to the condition of a still pond, and also that the water should 
have a fall from surface to surface not less in height than double 
the depth it runs over the notch. 
The observations should be repeated several times on different 
occasions, so as to obtain a fair average discharge. As a guide, 
it may be taken that a notch, 5 inches deep, Avill discharge 
about 100 gallons a minute. When the weir has no notch, the 
height of the water passing over the top can be ascertained by 
measuring the peg in the same manner, the step being placed 
level with the top of the weir. The product of the depth of 
water in inches (fi) passing over the weir, as ascertained from 
measurement on the step of the notch, multiplied by the square 
root of the depth and by the length (l") in inches, and a constant 
•43, gives the discharge (D) in cubic feet per minute or by 
2'67 for gallons, the formula being D=/iX\/A X ^ X "43. For 
example, a weir 72 inches long, with 2-inch overflow, will dis- 
charge 543"8 gallons or 87"3 cubic feet per minute. 
An approximate calculation, sufficient for preliminary pur- 
poses, may be made by holding a rule, marked in inches, on 
the lower edge of the weir, with the flat side opposed to the 
current. The difference in head to be allowed for and added to 
the reading will be from one-tenth to a quarter, according to the 
quantity of water passing over the weir.* 
The power of water to drive mills or any other hydraulic 
engines is derived from the weight of the water and the height 
* Neville's ' Hydraulic Tables.' Beardmore, ' Manual of Hydrology.' ' Practical 
Hydiaulics.' By Box. Spon and Co. A very useful little book, witli Tables. 
