# Page:EB1911 - Volume 14.djvu/65

DISCHARGE FROM ORIFICES]
53
HYDRAULICS

I 1S very suitable for measurements of this kind. If the How is variable, the head h must be recorded at equidistant intervals of time, say twice daily, and then for each I2-hour period f Scale Wet r L.; .;1 4.13:;:¢; -t ~:.i -e3\$§ -'- »-= -'Q—"' ': / I /Q —a -~ .».§ §§ =f:;/;=1 is =~<§ 1 ¢w1 i .r-'a- -'wary-1w1.., ,. ff: Ev:;;'7-'731: —::::°., f -'., 1 .. 1. I., ,, V - Q gy § &-f, :, ,, ,?, ,Zm, %, ¢., ,, , ~ 2 f Z f I i C

FIG. 66. 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, E. Sang 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 calculated by putting Q=r, 2, 5 . in the ordinary formulae 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 be inferred from the simplicity of the principle of the /, -T operation. A art 6; from the difficiilty Z* of selecting a suitable coefficient of W discharge, which need not be serious if the form of the j, weir and the nature of ilts crest aire propery atten ed to, other difficulties of FIG' 67 measurement arise. weir should be very accurately 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, The length of the and the adhesion of the water to the scale on which the head is measured, are liable to introduce errors. Upon a weir IO ft. long, with 1 ft. depth of water fiowing over, an error of Hoooth of a foot in measuring the "head, or an error of Hooth of a foot in measuring the length of the weir, would cause an error in computing the discharge of 2 cub. ft. per minute.-Hook Gauge.-For the determination of the surface level of water, the most accurate instrument is the hook gauge used first by U. Boyden of Boston, in 1840. It consists of a fixed frame with scale and vernier. In the instrument in fig. 68 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 -1 - *-1 S 3. o 1; EE fl e 1 2 é I e I e - f

| 2

l' 9 1 is @| ' é

• E

El 5

E 2 I 4 I E | é 3; t (D i g | 2 2 I é

Q l g 1' 2 I; §

• 2

- f 2 1 5 1 | nmmv,

| I |

E; -; Fig. 68 the'water surface and then raised by the screw, the moment of its reaching the water surface will be very distinctly marked, by the reflection 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