MEASUREMENT OF STREAMS. One of the many useful classes of work conducted by the United States Geological Survey; the measurement of streams having an industrial value. Wherever water is likely to be employed for power or irrigation the amount available is ascertained by that bureau. The volume varies greatly from season to season, and sometimes from year to year. Any calculation by the man ufacturer or farmer which does not take this variability into account would be misleading. Moreover, streams differ among themselves in straightness, the character of their beds and other particulars. Accurate measurement is still further complicated by the lack of uniformity in the movement in a given cross section. What might be true for one point would not be true for other points to the right or left of it, or at a higher or lower level.
The first step in the work is to place a gauge in a permanent position, and employ a suitable observer to read it every day. The gauge is graduated to feet and tenths. Usually it is up right. It may be laid over slanting, however. In that case, the spacing is widened, so as to give accurate results. One observation a day is made, and its results transmitted on a postal card to Washington. The second factor in the hydrographer's computation is the velocity of the stream, at different heights. To ascertain this only occasional measurements are neces sary. These are made by experts, but with their aid it is possible to prepare tables showing the total discharge of that particular stream for all the different gauge readings. After such tabu
lation is effected the maximum, minimum and mean flow for any month in the year can be told at a glance.
Velocity is ascertained with a meter lowered into the water. The instrument contains a tiny screw propeller, the number of whose rotations under the pressure of the fluid is automatically registered. Many types of meter have been de vised. The kind which the bureau prefers has an electric wire leading up to a buzzer in the operator's pocket. There a click or other audi ble sound indicates to him the speed of the screw. He counts the number of revolutions for some definite period, usually fifty seconds, records it in his notebook and moves along a short distance to make another observation.
If measurements are made at various depths along a vertical line, the results will not be equal. For this reason hydrographers have studied to find, if possible, a level that would fairly represent the average velocity for any vertical. In this way they can simplify the labor. It has been found that such an average can be secured by taking the speed at a depth of from six-tenths to two-thirds of the way down from the surface. To obtain a correct idea for the whole .stream, though, these tests must be made at a number of places between one shore and the other. No measurement at a single point can be trusted to be fairly repre sentative.