This is what engineers call a weir, a handy contrivance for measuring the flow of small streams. Experts have figured out an elaborate system of tables as to weirs. All we need to do now, in this rough survey, is to figure out the number of square inches of water flowing through this opening and falling on the other side. With a rule, measure the depth of the overflowing water, from the bottom of the opening to the top of the dead level of the water behind the board. Multiply this depth by the width of the opening, which will give the square inches of water escaping. For every square inch of this water escaping, engineers tell us that stream is capable of delivering, roughly, one cubic foot of water a minute. Thus, if the water is 8 inches deep in an opening 32 inches wide, then the number of cubic feet this stream is delivering each minute is 8 times 32, or 256 cubic feet a minute. So, a stream 32 inches wide, with a uniform depth of 8 inches running through our weir is capable of supplying the demands of the average farm in terms of electricity. Providing, of course, that the lay of the land is such that this water can be made to fall 10 feet into a water wheel. Go upstream and make a rough survey of the fall. In the majority of instances (unless this is some sluggish stream in a flat prairie) it will be found feasible to divert the stream from its main channel by means of a race--an artificial channel--and to convey it to a not far-distant spot where the necessary fall can be had at an angle of about 30 degrees from horizontal. If you find there is _twice_ as much water as you need for the amount of power you require, a five-foot fall will give the same result. Or, if there is only _one-half_ as much water as the 250 cubic feet specified, you can still obtain your theoretical five horsepower if the means are at hand for providing a fall of twenty feet instead of ten. Do not make the very common mistake of figuring that a stream is delivering a cubic foot a minute to each square inch of weir opening, simply because it _fills_ a certain opening. It is the excess water, falling _over_ the opening, after the stream has set back to a permanent dead level, that is to be measured. This farmer who spends an idle day measuring the flow of his brook with a notched board, may say here: "This is all very well. This is the spring of the year, when my brook is flowing at high-water mark. What am I going to do in the dry months of summer, when there are not 250 cubic feet of water escaping every minute?"