252 



o^--^ 



FiGUEE 85 



-Combination of constant and 

 currents. 



rotary 



If a constant (nontidal) current at the station has the direction 

 and strength O'O, the resultant of the tidal and constant currents at 

 the given time is O'P. Since P may be any point on the tidal curve, 

 the current curve of the resultant is the same as the curve of the rotary 



tidal current; but the pole is 

 shifted to 0' . This shift is in 

 the direction opposite to that 

 ot the constant current and 

 through the distance repre- 

 senting its velocity. If the 

 velocity of the constant cur- 

 rent exceeds the tidal, when 

 the latter has an opposing di- 

 rection, the pole shifts to a 

 point 0" , outside of the curve. 

 The direction of the resultant current then swings to and fro in 

 the limited arc between the tangent vectors 0"Pi and 0"Fi. 



The position of the pole of the diagram in figure 84 shows a pro- 

 nounced constant set of the current toward the north west at the 

 station. 



HARMONIC ANALYSIS AND PREDICTION OF TIDAL CURRENTS 



492. Current tables. — Advance information of the time at which 

 the currents in tidal waterways will change direction, and will reach 

 their strength at the flood and ebb ; and of the maximum velocities of 

 the surface currents in the navigation channel at each flood and ebb, 

 is of such value to navigators that yearly current tables giving this 

 information for the tidal waterways in and adjacent to the United 

 States are prepared and printed by the United States Coast and 

 Geodetic Survey. The tables give the predicted times of slack water 

 as the current turns from ebb to flood, or "slack before flood," and 

 from flood to ebb, or "slack before ebb," and the times and velocities 

 of the maximum flood and ebb currents, on each day of the year, 

 at a considerable number of reference stations. They also give the 

 corrections to be added to or subtracted from these times to obtain 

 the predicted times at a large number of secondary stations, and the 

 factors for reducing the predicted current strengths at a reference 

 station to those at the secondary stations. Most of the stations 

 listed are in confined channels at which the currents are of the revers- 

 ing type. Needless to say, the velocities in the tables are not the 

 mean velocities in the cross section of the waterway, which have 

 heretofore been dealt with, but are the surface velocities at definitely 

 located points or stations, so selected as to represent the currents 

 which will be encountered in the navigation of the fairway. 



493. Preparation oj current tables. — The fluctuations of the tidal 

 currents, like the tides, are caused by the tide producing forces of the 



