264 THE HYDROGRAPHY OF CECIL COUNTY 



vey, in conjunction with the Maryland Geological Survey, and sys- 

 tematic measurements have been made at Rowlandsville. Meas- 

 urements of flow have been made with a small Price current 

 meter at regular intervals according to the customary methods of the 

 Survey. A gage of some kind is established by which the height of 

 the river can be determined and recorded daily, or several times a day. 

 This gage reads to the nearest tenth, so that for each day of the year 

 the stage of the river is known. 



Frequent measurements are made by the hydrographer of the flow, 

 usually at some bridge across the stream. Careful soundings are 

 made, so that the area of the cross-section can be computed. This 

 cross-section is then divided into smaller sections of 5 or 10 feet in 

 width by marks upon the guard-rail of the bridge. By knowing the 

 width and average depth it is possible to compute the area of each of 

 the partial sections, and the velocity in each can be determined with 

 the current meter. 



This instrument, designed on the principles of the propeller of a 

 ship, is suspended from the bridge in the water and held perfectly 

 quiet. The flowing water turns the propeller wheel, its speed being 

 dependent upon the velocity of the water. The number of revolu- 

 tions in a certain short period of time is noted by the hydrographer. 

 Each revolution is recorded by a small electric buzzer, through which 

 a current is passed when the revolving wheel of the current meter 

 makes each revolution. The hydrographer, watch in hand, counts 

 the number of breaks in the current by the sounds of the buzzer and 

 records it in his note book. Usually a run of 50 seconds is made for 

 convenience in computation, and each run is repeated as a check. 

 Electric recorders have been used for this purpose, but it has been 

 found that better results can be obtained by the use of the buzzer. 



Before using the meter in the field it is rated, so that the relation 

 between the velocity of the current and the number of revolutions of 

 the wheel is known. This rating is done at "Washington by passing 

 the meter through still water at various known speeds and noting the 

 number of turns made by the wheel in a distance of 100 feet. The 

 results of a number of such runs are plotted on cross-section paper, 



