CURRENT METERS 351 



to clog are obvious, and the mechanically operated meter is becoming 

 obsolete. 



The instrument is previously calibrated by towing at known velocities 

 through still water, the number of revolutions corresponding to these 

 velocities being recorded. It has the disadvantages that it cannot be 

 used where floating grass or weed is prevalent, and that it requires 

 rating at frequent intervals. Further, it cannot be used at very low 

 velocities. The minimum permissible velocity depends on the type of 

 meter, but in general varies from 3 to 6 inches per second. 



There, are two methods of using the meter. In the first, the " point ' 



FIG. 156. Price Current Meter. 



method, it is held successively at certain points in a cross section. In 

 a shallow stream this may be done by clamping it to a staff which 

 is carried by an observer in waders, and which is held vertically at the 

 required points, with one end resting on the bed of the stream. In 

 deeper streams it is attached to a heavy sinker and is suspended from a 

 convenient bridge or cable placed across the stream where possible, or 

 from an outrigger fixed to an anchored boat where the width precludes 

 this. When the " point " method is used, the meter may either be held, 

 (1) at several equidistant points in certain equidistant verticals, the 



