HYDRAULICS 



359 



number and the velocity of current, called the rating of the 

 instrument, is usually effected by drawing the meter at a given 

 speed through still water. The part s is a rudder and the 

 part B a ballast for use in very deep water. The approximate 

 mean velocity of flow at a cross-section of a str.eam may be 

 determined by measuring the velocity of the depth at .6 below 

 the surface at the deepest part of the cross-section. When 

 accurate results are required, measurements should be taken at 

 different parts of the section as well as at different depths of 

 the same section and the average calculated. The ordinary 

 method of procedure is as follows: 



A range at right angles to the stream is selected (see Fig. 2) 

 and divided into any desired number of parts. Soundings are 

 taken along the points of division, and at the same points the 



FIG. 2 



mean velocities are determined by moving the meter vertically 

 at a uniform rate from the surface of the water to the bottom 

 and back to the surface. The mean velocity of a division 

 multiplied by the corresponding area gives the partial discharge 

 of that division. The sum of the partial discharges is the total 

 discharge of the river. 



Determination of Discharge by Floats. The discharge of 

 streams is best determined by means of rod floats, which are 

 wooden or hollow tin cylinders weighted at the lower end. They 

 should be placed as near the bottom of the stream as possible. 

 A suitable portion of the stream between two cross-sections at 

 right angles to it is selected. The sections are divided into a 

 -uitable number of parts, soundings are taken at each division 



