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 × 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 inches wide and 6-1/4 deep, giving 189 cubic feet of water a minute--_Quantity._ Since such a stream is of common occurrence on thousands of farms, let us analyze briefly its possibilities for power: One hundred and eighty-nine cubic feet of water weighs 189 × 62.5 pounds = 11,812.5 pounds. Drop this weight one foot, and we have 11,812.5 foot-pounds. Drop it 3 feet and we have 11,812 × 3 = 35,437.5 foot-pounds. Since 33,000 foot-pounds exerted in one minute is one horsepower, we have here a little more than one horsepower. For simplicity let us call it a horsepower. [Illustration: Detail of a water-power plant, showing setting of wheel, and dynamo connection]