CURRENT METERS 



853 



TO feet and the velocities from *4 to 1/5 feet per second. For a gravelly 

 bed the meter should be held with its centre from 3 to 4 inches above the 

 bottom and about 2 inches below the surface, while with a smooth bed 

 each distance should be about 2 inches. With depths between 5 and 10 

 feet, the mean of velocities *5 feet above the bottom and '5 feet below the 

 surface gave results too low by as much as 30 per cent. In such cases 

 the mean of readings at two-tenths and at eight-tenths the depth gives 

 good results. 



In the " integration " method, the meter is kept in motion during the 

 whole period of its immersion. It may either be moved uniformly from 

 the surface to the bottom of the stream in a series of vertical lines ; 

 diagonally across from one side to the other, at the same time being 

 moved from the surface to the bottom several times ; or across the stream 

 at a given depth. The recorded velocity is then taken as the mean 

 for the particular vertical or for the whole section as the case may be. 

 Although an observation by this method can be carried out in consider- 

 ably less time than by the point method, the results are not nearly so 

 accurate. The velocity recorded being the resultant of the velocities of 

 the meter and of the water is always higher than the true velocity, the error 

 increasing with the speed of movement of the meter and also increasing 

 as the velocity of flow diminishes. It is only to be recommended where 

 a stream is rising or falling rapidly and where in consequence the speed 

 with which the observations can be made is a great advantage. 



Simultaneously with the velocity observations, soundings should be 

 taken from which the cross section of the stream may be obtained. In 

 a narrow stream these should be taken at intervals of from 2 to 5 feet, 

 while where the breadth exceeds 100 feet, they should be taken at 

 intervals of from 10 to 25 feet, depending on the roughness of the bed. 



Field Notes. The following shows the method adopted for entering up 

 field observations and computing mean velocities in the case where 

 velocities are measured at several points on a cross section. 



[Gauging made January 23, 1904, by B.S.D. Meter No. 349, on Dan River, Madison, N.C. 

 Gauge height ; beginning 2-10 feet. ; end 2-26 feet ; river rising.] 



H.A. 



A A 



