224 



The range of the tide decreases from 6 feet at the entrance to 5.0 

 feet at a point 40,000 feet (about 8 miles) up the canal, and thence 

 increases to 5.8 feet at the head of the canal. High water at the 

 head of the canal is 60°. 8/28. 98 = 2.1 mean solar hours later than at 

 the entrance. The rate at which the tide progresses up the canal is 

 far from uniform. In the first subsection next the entrance it pro- 

 gresses at the rate of 8.2 feet per second, while in the upper 40,000 

 feet it progresses at an average rate of over 200 feet per second. The 

 rate of advance of a progressive wave in a canal of the given depth 

 would be -y 16fif=22.7 feet per second. 



The instantaneous profiles in the canal at successive lunar hours 

 are shown in figure 77. 



In an even longer canal of the same depth the rate of progress of 

 the tide is found to decrease slowly from the entrance for some dis- 

 tance up the channel and thence increase rapidly toward the head. 



431. The primary current at the entrance reaches a maximum of 

 3.2 feet per second, and the strength of the positive current occurs 



^ 



3-^ 



Sta 140 ISO lOO 80 60 40 20 O 



Figure 77. — Instantaneous profiles in closed canal 140,000 feet long and 16 feet mean depth. 



1.5 mean solar hours before high water, or 1.6 hom's after mid tide. 

 The strength of the current decreases nearly uniformly to the head 

 of the canal. Near the head of the canal the strength of the positive 

 current is 30 minutes later than at the entrance, and occurs nearly 

 at midtide at the head of the canal. 



Considerable deformations of the primary currents are to be ex- 

 pected in so long and shallow canal; but the many successive approxi- 

 mations necessary to bring the deformations of the tides and currents 

 into concordance would render their computation excessively laborious. 



432. Comimtation jor canal of varying cross section. — If the surface 

 width and the mean depth of a closed canal are not the same through- 

 out, the values of the coefficients, 7, p, and la/g are determined for 

 each subsection, and the form of the computations is modified in the 



