ordinary foot
rule, or a folding rule, will give results sufficiently accurate for
the water prospector in this instance. The depth should be measured
not at the opening itself, but a short distance back of the opening,
where the water is setting at a dead level and is moving very slowly.

With this weir, every square inch of water flowing through the opening
indicates roughly one cubic foot of water a minute. Thus if the
opening is 10 inches wide and the water flowing through it is 5 inches
deep, the number of cubic feet a minute the stream is delivering is 10
x 5 = 50 square inches = 50 cubic feet a minute. This is a very small
stream; yet, if it could be made to fall through a water wheel 10 feet
below a pond or reservoir, it would exert a continuous pressure of
30,000 pounds per minute on the blades of the wheel--nearly one
theoretical horsepower.

This estimate of one cubic foot to each square inch is a very rough
approximation. Engineers have developed many complicated formulas for
determining the flow of water through weirs, taking into account fine
variations that the farm prospector need not heed. The so-called
Francis formula, developed by a long series of actual experiments at
Lowell, Mass., in 1852 by Mr. James B. Francis, with weirs 10 feet
long and 5 feet 2 inches high, is standard for these calculations and
is expressed (for those who desire to use it for special purposes) as
follows:

Q = 3.33 L H^(3/2) or, Q = 3.33 L H sqrt(H),

in which Q means _quantity_ of water in cubic feet per second, L is
length of opening, in feet; and H is height of opening in feet.

The following table is figured according to the Francis formula, and
gives the discharge in cubic feet per minute, for openings one inch
wide:

TABLE OF WEIRS

Inches 0 1/4 1/2 3/4
1 0.403 0.563 0.740 0.966
2 1.141 1.360 1.593 1.838
3 2.094 2.361 2.639 2.927
4 3.225 3.531 3.848 4.173
5 4.506 4.849 5.200 5.558
6 5.925 6.298 6.681 7.071
7 7.465 7.869 8.280 8.697
8 9.121 9.552 9.990 10.427
9 10.884 11.340 11.804 12.272
10 12.747 13.228 13.716 14.208
11 14.707 15.211 15.721 16.236
12 16.757 1