155 



outward, and therefore intrinsically negative in sign, the difference 

 between the ordinates of the instantaneous tidal and the fresh-water 

 discharge is the total discharge at the instant. These total discharges 

 are then the ordinates measured from the line of the fresh-water 

 discharge. 



The area of the cross section at the Sliip John at the heights shown 

 by the tide curve is plotted on the diagram for the station. By taking 



,/2<5^ 



13 14 



Mean Solar Hour 



Figure 50.— Discharges and velocities in Delaware River at Philadelphia, from mean cubature. 



off from the diagram the total discharge and the cross sectional area at 

 a given instant, a refined value of the current velocity at that instant 

 may be computed. 



313. An examination of these curves shows that at Ship John Light 

 the average duration of the rise of the tide is about 1 hour less than 

 the duration of the fall; and that the duration of the ebb current is 

 nearly 45 minutes longer than the duration of the flood. At Pliila- 

 delphia, the average duration of the rise of the tide is about 2 hours 

 less than the average duration of the fall; and the duration of the ebb 



192750—40- 



-11 



