7.283 17.816 18.352
13 18.895 19.445 19.996 20.558
14 21.116 21.684 22.258 22.835
15 23.418 24.007 24.600 25.195
16 25.800 26.406 27.019 27.634
17 28.256 28.881 29.512 30.145
18 30.785 31.429 32.075 32.733

Thus, let us say, our weir has an opening 30 inches wide, and the
water overflows through the opening at a uniform depth of 6-1/4
inches, when measured a few inches behind the board at a point before
the overflow curve begins. Run down the first column on the left to
"6", and cross over to the second column to the right, headed "1/4".
This gives the number of cubic feet per minute for this depth one inch
wide, as 6.298. Since the weir is 30 inches wide, multiply 6.298 x 30
= 188.94--or, say, 189 cubic feet per minute.

Once the weir is set, it is the work of but a moment to find out the
quantity of water a stream is delivering, simply by referring to the
above table.

_Another Method of Measuring a Stream_

Weirs are for use in small streams. For larger streams, where the
construction of a weir would be difficult, the U. S. Geological Survey
engineers recommend the following simple method:

Choose a place where the channel is straight for 100 or 200 feet, and
has a nearly constant depth and width; lay off on the bank a line 50
or 100 feet in length. Throw small chips into the stream, and measure
the time in seconds they take to travel the distance laid off on the
bank. This gives the surface velocity of the water. Multiply the
average of several such tests by 0.80, which will give very nearly the
mean velocity. Then it is necessary to find the cross-section of the
flowing water (its average depth multiplied by width), and this
number, in square feet, multiplied by the velocity in feet per second,
will give the number of cubic feet the stream is delivering each
second. Multiplied by 60 gives cubic feet a minute.

_Figuring a Stream's Horsepower_

By one of the above simple methods, the problem of _Quantity_ can
easily be determined. The next problem is to determine what _Head_ can
be obtained. _Head_ is the distance in feet the water may be made to
fall, from the Source of Supply, to the water wheel itself. The power
of water is directly proportional to _head_, just as it is directly
proportional to _quantity_. Thus the typical weir measured above was
30