CURRENT METERS 



855 



be computed somewhat more accurately by considering the discharge 

 between any two such verticals as being represented by the volume of 

 the solid having these curves bounding opposite parallel sides as shown in 

 Fig. 157. For example, the discharge between the verticals 2 and 3 in 

 this figure is given by 



+ V ^2 /<2 3 



+ 



c.f.s. 



where r 2 and F 3 are mean velocities in the verticals 2 and 3, and where 

 /? 2 and h 3 are the corresponding soundings, 2 d 3 being the distance between 

 the verticals. The discharge between the two end soundings is then 

 given by the sum of such terms as the above between these soundings 



FIG. 157. 



To this must be added the discharge over these sections outside the end 

 soundings, which is given by 



J { vi h! X <h + r 5 h 5 X & } c.f.s. 



Calibration of Current Meters. In rating a meter it is usually suspended 

 from a car or a boat, and is towed with a uniform velocity through still 

 water at a depth of 2 or 3 feet. The length of a run varies from 100 to 

 300 feet, with sufficient of a starting run to attain a steady velocity 

 before entering the measured length. It is moved in either direction 

 from end to end of the run to eliminate the effect of a current in either 

 direction, and the time of the run and the number of revolutions of the 

 meter are recorded by means of a chronograph. 



The range of velocities employed in rating should be those for which 

 the meter is to be used, and no attempt should be made to extend the 

 rating table beyond its experimental limits. 



When in use the meter may either be suspended from a cable, in which 

 case its axis is free to move about both a vertical and a horizontal axis, 

 or it may be fixed to a vertical rod in such a way as to remove the second 

 degree of freedom. Experiments indicate that the same rating table iu 



A A 2 



