CHEMICAL ANALYSIS 



23 



and weighted with shot or mercury, so as to sink about three- 

 quarters of its depth, the upper end carrying a flag to make it 

 conspicuous. Several preHminary trials should be made as to 

 the length of the tube that can pass without obstruction, and in 

 some cases a string may be attached for recovery, with care to 

 avoid error. Formulae for calculating the mean velocity from 

 the surface velocity will be found in engineering books ; in 

 general, the volume of discharge is obtained by multiplying the 

 mean sectional area of the channel (found by measuring at 

 several parts) by the mean velocity in feet per second. 



A method which dispenses with formulae, and is quite accurate 

 where applicable, is to measure the time in seconds required to 

 fill a pail or zinc bath of known capacity. In a recent instance 

 my average gauging by the pail was 15,886 gallons per twenty- 

 four hours ; the official figures afterwards received were 15,571. 



A simple rule to remember is that cubic feet per minute 

 X 9,000 = gallons per twenty-four hours. 



An effluent is easier to measure on account of the absence of 



Fig. I. — V Notch. 



solid matter. It may be made to flow through a V notch at an 

 angle of 90 degrees in the side of a tank. The area of the orifice 

 will be half the square of the side from the vertex to the water 

 level. The amount of the effluent can be calculated, or may 

 be determined experimentally, and tables prepared giving 

 volumes of flow corresponding to the level of liquid in eighths 

 of an inch.i The actual V notch should be a thin plate of zinc 

 or thickly-galvanized iron ; or, if of wood, it must be sawn to 

 a clean bevel, with the sharp edge up-stream. 



This form of notch has an advantage over a square weir, 

 since, however small the flow of water, there is always a 

 readable quantity. The variation in the area of water flowing 



^ See p. 25, 



