120 MEASUREMENT OF FLOWING WATER.
multiplying
the mean velocity due to the grade by the average cross section is
sufficiently accurate. The discharge of small streams is obtained more
exactly by filling vessels of known capacity.
Formula for Discharge over Weirs.—In
gauging large quantities of water over weirs Fteley and Stearns's
general formula can be used for the discharge over the simplest form of
sharp-crested weir, unaffected by end contractions or velocity of
approach. If these conditions exist the corrections for them must be
made separately.*
The formula is
Q
is the quantity in cubic feet per second, L the length of the weir, and
H the depth on the weir corrected for velocity of approach. This
formula does not apply to any depth of the weir less than 0.07 feet.
Discharge through Triangular Notches.—The
right-angled triangular notch of thin sheet iron is a very convenient
way of measuring the discharge of water. According to Prof. Thompson's
experiments, the dis-
charge in cubic feet per second = head^ (in inches) X 0.0051.
To
use the notch, construct a weir box, O, with a triangular notch, Y,
made of iron, fitted in one end. The edge of the notch must be sharp
and bevelled out, and the inside face must be placed at right angles to
the surface of the water, M. Place in the box baffle boards or strips,
K K, to render the surface of the water near the point A uniform or
still (A is taken about 18 to 24 inches back from the weir plate Y).
Place a spirit-level or straight-edge C on the weir plate at E; measure
the distance at A from C to surface of water. Subtract this from H, and
find the difference in column marked h of Table VII. Opposite h,
* See " Transactions American Society Civil Engineers," vol. xii. p. 32.
