CHAPTER IX.

MEASUREMENT OF FLOWING WATER.*

Weirs. The direct measurement of flowing water in a stream or channel can be made in various ways. Occasionally gauge wheels are used, but the method is expensive. Gauging by rectangular overfalls (weirs) of certain dimensions and under certain circumstances gives results within one per cent. of absolute exactitude (Francis' formula).

In employing this method the height above the crest of the surface of still water, some little distance back from the weir, must be carefully measured. It is also desirable that there should be no considerable current to the water at the place of measurement.

Orifices.-Flowing water is measured also by its discharge under pressure through an aperture of regular section. Though it is not theoretically correct, there will be no practical error in assuming the average head to be from the centre of the aperture when the width is considerably less than the height of the water above the top of the opening.

Open Channels.-The measurement of the surface velocity of water passing through a flume or canal of uniform size can be used to determine its discharge, and in some cases the simple calculation of discharge made by

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* For details on the subject of the measurement of water see "The Mechanics of Engineering," by Julius Weisbach, translated by E. B. Coxe; Francis' "Lowell Hydraulics"; port Mississippi River," by Humphreys and Abbot; "Hydraulic Manual," by Louis D'A. Jackson; "The New Formulæ for the Mean Velocity of Discharge of Rivers and Canals," by W. R. Kutter; "Hydraulic Tables," by Thos. Higham; "A Treatise on Water-Supply Engineering," by J. T. Fanning; Experiments on the Flow of Water," by A. Fteley and E. P. Stearns, vol. xii. “Transactions of the American Society of Civil Engineers."