12 U. S. COAST AND GEODETIC SURVEY 



of the major axis. If the major and minor axes are nearly equal the 

 ellipse will be nearly circular; if they differ greatly the ellipse will be 

 flattened. In the northern hemisphere the direction of rotation of 

 the rotary current is, as a rule, with the hands of a clock, while in 

 the southern hemisphere it is counterclockwise. Local hydrographic 

 features may bring about a reversal of this general rule. 



Rotary tidal currents are subject to the periodic variations found 

 in tides and reversing currents. These variations are related to the 

 changes in the phase, parallax, and declination of the moon. At 

 times of full and new moon the velocity of the rotary current is 

 greater than the average, while at the times of the moon's first and 

 third quarters the velocities are less than the average. Likewise 

 when the moon is in perigee, stronger currents occur, while when the 

 moon is in apogee the currents are weaker. In general it may be 

 taken that the percentage of increase or decrease in the velocity of 

 the current in response to changes in phase and parallax is the same 

 as the like increase or decrease in the local range of the tide. 



In response to changes in the declination of the moon the rotary 

 current exhibits diurnal inequality like the tide and reversing current. 

 This manifests itself as a difi^erence between morning and afternoon 

 current ellipses. When the moon is on the Equator the two current 

 ellipses of a day are much alike, but when the moon is near its maximum 

 semimonthly declination the two current ellipses exhibit differences, 

 principally in velocity. 



Like tides and reversing currents, rotary tidal currents may be 

 grouped under the three types of semidaily, daily, and mixed. The 

 semidaily type of rotary current is one which exhibits two full cycles 

 within a tidal day, morning and afternoon currents differing but 

 little. The daily type is one in which but one cycle occurs in a day; 

 and the mixed type is one which exhibits two cycles within a day, 

 but with considerable differences between morning and afternoon 

 currents. 



EFFECTS OF NONTIDAL CURRENTS ON ROTARY CURRENTS 



In addition to the periodic variations to which rotary tidal currents 

 are subject, they also exhibit fluctuations arising from the effects of 

 nontidal currents. These effects can most conveniently be studied 

 diagrammatically. 



Figure 6 represents the purely rotary tidal current at Nantucket 

 Shoals Lightship. Now suppose that on a given day a wind begins 

 blowing from the northeast such that it produces a wind-driven cur- 

 rent of half a knot in a southwesterly direction. For that day, 

 obviously, the velocity and direction of the current at Nantucket 

 Shoals Lightship will be different than represented in figure 6. At 

 2 hours before low water at Boston, for example, the tidal current 

 sets southwesterly with a velocity of 0.85 knot on the average; but 

 with a nontidal current of 0.5 knot due to the wind setting in the 

 same direction, the velocity of tha current now experienced will 

 be 0.85 + 0.50 = 1.35 Icnots, setting southwesterly. On th3 other 

 hand, about 2 hours before high water, the current will be setting 

 0.85 — 0.50 = 0.35 knot northeasterly. 



The current conditions at this time may be completely represented 

 by changing the origin of the hourly velocity and direction lines in 

 figure 6 from the center to a point 0.5 knot northeasterly of its pre- 



