MEASUREMENT OF FLOWING WATER.                 127
	This is called Torricelu's theorem, which supposes in­definitely small orifices with thin sides, and assumes that the upper surface of the water and the orifices are under the same conditions as regards atmospheric pressure. Conditions and size of sectional area of the aperture, fric­tion, resistance of the air to motion, and pressure of the atmosphere are all neglected. The value of g varies in different latitudes, but for all practical purposes is taken as equal to 32.2.
	Flow of Water in Open Channels.—There is no generally accepted formula for determining the velocity of water in open channels. The tables based on the old formulas published prior to the works of D'Arcy and Ba-zin in France, and of Humphreys and Abbot in the United States, being founded on data which ignored the important factor of the nature of the bed and the sides of the channel, have proved unsatisfactory. Hydraulic en- * See professional papers, Corps of Engineers U. S. A., No. 12, page 26.