216 



415. Second approximation.- — The corrections applied to the veloci- 

 ties in the subsections of the canal must change, to some extent, the 

 distribution of the total surface head between the subsections. A 

 second adjustment may be made by computing the surface heads 

 produced by the corrected velocities in each subsection of the canal, at 



18 O 



Standard Time 



Computed primary current 

 Adjusted current 



Recorded currents o o o 

 Figure 71.— Computed and observed curreuts, Chesapeake and Delaware Canal, November 27-28, 1928 



the adopted half lunar hour intervals. These heads are determined 

 from the equation: 



hs-\-h,)-\-ha+hf=0 



by computing the values of hv, ha, and hf from the corrected velocities. 

 The surface heads so computed may then be adjusted to the heads 

 estabhshed by the selected entrance tides, corrected tidal heights at 

 the ends of the subsections derived therefrom, and a second determina- 

 tion of the velocities made from the corrected tides. The procedure 

 is too laborious to be warranted in any ordinary case. The results of 

 its application to the tides and currents in the canal chosen for the first 

 example, in which the entrance tides were taken with a simple harmonic 



