213 



computations which are not here repeated, give the corrected velocities 

 shown in column (3) of the following tabulation, from which the dis- 

 charges are computed. 



The tidal heights at station 69, column (4), are the means of the 

 adjusted heights at stations 61 and 77. From a sheet of typical cross 

 sections, the area of the water prism at station 69 is found to be 1,900 

 square feet at tide and 2,944 square feet at a 6-foot tide. The areas, 

 X, of the cross section at the tidal heights shown in column (4) are 

 taken off a straight line diagram and entered in column (5). These 

 areas, multiplied by the velocities in colum.n (3), give the discharges, 

 Q, in cubic feet per second at half lunar hour intervals, shown in column 

 (6). These, and subsequent computations, are by slide rule. The 

 discharges through the rest of the 12 hour tidal cycle are computed in 

 the same way. 



411. Discharges and velocities at other stations. — The adjusted dis- 

 charges and velocities at any other station on the canal are determined 

 from the discharge at station 69 by the cubature of the adjusted tides 

 through successive intervening reaches. As shown by equation (276), 

 paragraph 368, the increase, AQ, in the discharge between two succes- 

 sive velocity stations is: 



AQ=-U()y/bt 



in which U is the area of the water surface between the stations and 

 dyldi is the rate at which the average tidal height between the stations 

 IS increasmg with the time. For sufficiently small increments of 

 time, At, this equation may be written: 



Q=- U Ay I At 



in which TJ is the area of the water surface at a given time. At is the 

 selected time interval, and Ay may be taken as the mean increase in 

 the average tide between the stations during the preceding and follow- 

 ing intervals. In the present case. At is the half lunar hour of 1,863 

 seconds. 



