218 PROCESSES FOR GAUGING STREAMS. 



discharge can be computed. The sum of the partial dis- 

 charges will be the total discharge of the stream. The fol- 

 lowing formula* is convenient for determining the ratio of 

 the surface velocity to the mean velocity in the same verti- 

 cal. Let v be the mean and V the surface velocity in any 

 given vertical longitudinal section, the depth of which is h. 



v_ _ 1 + 0.1478^ 

 v ~ 1 + 0.2216VA* 



Sen. In the gaugings of the Mississippi, it was found 

 that the mid-depth velocity differed by only a very small 

 quantity from the mean velocity in the vertical section, and 

 it was uninfluenced by wind. If therefore a series of mid- 

 depth velocities are determined, they may be taken to be 

 the mean velocities of the compartments in which they 

 occur, and no formula of reduction is necessary. 



118. Processes for Gauging Streams. The dis- 

 charge of large creeks, canals, and rivers, can be measured 

 only by means of hydrometers, which are instruments for 

 indicating the velocity. The simplest of these instruments 

 are surf ace floats ; these are convenient for determining the 

 surface velocities of a stream, though their use is difficult 

 near the banks. Any floating body can be used for this 

 purpose ; but it is safer to employ bodies of medium size, 

 and of but little less specific gravity than the water itself. 

 Very large bodies do not easily assume the velocity of the 

 water, and very small bodies, especially when they project 

 much above the surface of the water, are easily disturbed in 

 their motion by accidental circumstances, such as wind, etc. 



The floats may be small balls of wood, of wax; or of hol- 

 low metal, so loaded as to float nearly flush with the water 

 surface. To make them visible, they may have a vertical 

 painted stem. In experiments on the Seine, cork balls 1| 



* Given by Eyner in Erbkam's Zeitschrift for 1875. 



