GENERAL EXPLANATION 9 
20. In the semidiurnal tidal movement, the direction of the rotary current makes 
two complete cycles each lunar day, the velocity variations being approximately the 
same for each cycle. In the diurnal movement there is only one cycle during the day. 
In the mixed type of tide there is a tendency for two cycles each day with marked differ- 
ences in velocity variations, but in many cases the direction of the current makes a 
complete turn around the compass only during the cycle of greater velocity and moves 
through a limited are during the cycle of lesser velocity. 
21. In representing rotary currents graphically there are three elements to con- 
sider—time, velocity, and direction. The data may be plotted with rectilinear coordi- 
nates to show the relation of the velocity to time with a separate graph to show the 
relation of the direction to time; or they may be plotted with polar coordinates to 
show the relation of velocity and direction with the radii vectores at specified time 
intervals which are indicated on the graph. The plotting of velocities with rectilinear 
coordinates is similar to that used for reversing currents, except that for the rotary cur- 
rents all velocities are considered as positive regardless of direction. 
22. The polar coordinate system is most generally used for plotting mean values 
where the current has been referred to hours of the daily or semidaily tidal cycle, the 
initial references usually being the times of high and low water or the principal current 
phases at some standard station. In its simplest form such a graph approximates an 
ellipse in shape. Figure 10 represents the mean semidiurnal tidal current at Nantucket 
Shoals Lightship with the radii vectores referred to the hours of the tide at Boston. 
The letters ‘“H”’ and “L” signify respectively the times of high and low water at Boston 
Each radius vector indicates by its length and direction the velocity and direction of 
the current at the hour designated. Figure 11 represents the mixed type of current at 
LL+yh 
LL 
1 knot 
FicurE 11.—Mean current curve for Swiftsure Bank Lightship, referred to tides at Astoria. 
