Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/493

477 HYDRAULICS,] 51. Measurement of the Flow in Streams. To determine the quantity of water flowing off the ground in small streams, which is available for water supply or for obtaining water power, small tem- 64. porary weirs are often used. These may be formed of planks supported by piles and puddled to prevent leakage. The measurement of the head may be made by a thin-edged scale at a short distance behind the weir, where the water surface has not begun to slope down to the weir and where the velocity of approach is not high. The measure ments are conveniently made from a short pile driven into the bed of Scale Weir 65. the river, accurately level with the crest of the weir (fig. 65). Then if at any moment the head is h, the discharge is, for a rectangular notch of breadth b, where c = 62 ; or, better, the formula in 38 may be used. Gauging weirs are most commonly in the form of rectangular notches ; and care should be taken that the crest is accurately hori- xontal, and that the weir is normal to the direction of flow of the stream. If the planks are thick, they should be bevelled (fig. 66), and then the edge may be protected by a metal plate about t Vth inch thick to secure the requisite accuracy of form and sharpness of edge. In permanent gauging weirs a cast steel plate is sometimes used to form the edge of the weir crest. The weir should be large enough to discharge the maximum volume flowing in the stream, and at the same time it is desirable that the minimum head should not be too small (say half a foot) to decrease the effects of errors of measurement. The section of the jet over the weir should not exceed one-fifth the section of the stream behind the weir, or the velocity of approach will need to be taken into account. A triangular notch is very suitable for measurements &amp;lt;&amp;gt;f this kind. If the flow is variable, the head h must be recorded at equidistant intervals of time, say twice daily, and then for each 12 hour period the discharge must be calculated for the mean of the heads at the beginning and end of the time. As this involves a good deal of troublesome calculation, Mr Sang lias proposed to use a scale so graduated as to read off the discharge in cubic feet per second. The lengths of the principal graduations of such a scale are easily calcu lated by putting Q = l, 2, 3 .... in the ordinary formula for notches ; the intermediate graduations may be taken accurately enough by subdividing equally the distances between the principal graduations. The accurate measurement of the discharge of a stream by means of a. weir is, however, in practice, rather more difficult than might V J inferred from the simplicity of the principle of the operation. L 477 Apait from the difficulty of selecting a suitable coefficient of dis charge, which need not be serious if the form of the weir and the nature of its crest are properly attended to, other difficulties of measurement arise. The length of the weir should be very accu rately determined, and if the weir is rectangular its deviations from exactness of level should be tested. Then the agitation of the water, the ripple on its surface, and the adhesion of the water to the scale on which the head is measured, are liable to introduce errors. Upon a weir 10 feet long, with 1 foot depth of water flowing over an error of l-1000th of a foot in measuring the head, or an error of 1-1 00th of a foot in measuring the length of the weir, would cause an error in computing the discharge of 2 cubic feet per minute. 52. Hook Gauge. For the determination of the surface level of water, the most accurate instrument is the hook gauge used first by Mr U. Boyden of Boston, in 1840. It consists of a fixed frame with scale and vernier. In the instru ment in fig. 67 the vernier is fixed to the frame, and the scale slides vertically. The scale carries at its lower end a hook with a fine point, and the scale can be raised or lowered by a fine pitched screw. If the hook is depressed below the water surface and then raised by the screw, the moment of its reaching the water surface will be very dis tinctly marked, by the reflexion from a small capillary elevation of the water surface over the point of the hook. In ordinary light, differences of level of the water of 001 of a foot are easily detected by the hook gauge. If such a gauge is used to determine the heads at a weir, the hook should first be set accurately level with the weir crest, and a reading taken. Then the difference of the reading at the water surface and that for the weir crest will be the head at the weir. 53. Modules used in Irrigation. In distributing water for irrigation, the charge for the water may be simply assessed on the area of the land irrigated for each consumer, a method followed in India ; or a re gulated quantity of water may be given to each consumer, and the charge may be made proportional to the quantity of water supplied, n method employed for a long time in Italy and other parts of Europe. To deliver a regulated quantity of ,,. v fi ~ water from the irrigation channel, arrangements termed modules are used. These are constructions intended to maintain a constant or approximately constant head above an orifice of fixed size, or to regulate the size of the orifice so as to give a constant discharge, notwithstanding the variation of level in the irrigating channel. 54. Italian Module. The Italian modules are masonry construc tions, consisting of a regulating chamber, to which water is admitted by an adjustable sluice from the canal. At the other end of the chamber is an orifice in a thin flagstone of fixed size. By means of the adjustable sluice a tolerably constant head above the fixed orifice is maintained, and therefore there is a nearly constant dis charge of ascertainable amount through the orifice, into the channel leading to the fields which are to be irrigated. &quot;Tr Regulating 1 Channel tc, fields Fig. 68. In fig. 68 A is the adjustable sluice by which water is admitted to the regulating chamber, B is the fixed orifice through which the water is discharged. The sluice A is adjusted from time to timely the canal officers, so as to bring the level of the water in the re^ulat-