MARYLAND GEOLOGICAL SURVEY 265 



the revolutions per second being plotted as abscissae, and the velocities 

 per second as ordinates. A smooth curve is then drawn through these 

 points, and any desired intermediate velocities can then be interpo- 

 lated. A rating table is then constructed, taking the data from this 

 curve. 



When the measurement of a stream is made and the number of 

 revolutions of the wheel noted, the velocity of the flow can be deter- 

 mined at once by reference to this table. Knowing the velocity in 

 each of the smaller sections across the stream, and the area of each, 

 the quantity of water passing can be found by multiplying the two 

 together, and the total flow past the station is found by the summa- 

 tion of the flow through all partial areas. In this way the actual flow 

 of the river can be found on various days. A list of measurements 

 made in this way of the Octoraro is given on one of the following 

 pages. 



Owing to the expense of making such measurements it is not prac- 

 ticable to make them every day in the year on the same stream, and 

 yet it is important that the flow of the stream for each day in the 

 year should be known. To this end the river height is read each day 

 on the gage as described above. By knowing this reading at the 

 time of each measurement made with the meter, the relation between 

 the gage height and the discharge of the stream is determined. A 

 station rating curve is drawn with gage height as abscissae, and dis- 

 charges in cubic feet per second as ordinates. 



The result of each measurement is plotted on cross-section paper 

 and a smooth curve drawn through these points. From this curve 

 the approximate discharge in cubic feet per second can be read at 

 once for any assumed gage height, so that with the gage height known 

 the approximate discharge can be determined for each day in the 

 year. Computations of this kind have been made on Octoraro Creek. 

 The results are given below in condensed form, showing the maxi- 

 mum, minimum and mean discharges for each month, with the run- 

 off in second-feet per square mile, and depth in inches on the water- 

 shed. The record of the gage heights is also given, and the station 

 rating table. 



