10 



U. S. COAST AND GEODETIC SURVEY 



A characteristic feature of the rotary current is the absence of 

 slack water. Although the current generally varies from hour to 

 hour, this variation from greatest current to least current and back 

 again to greatest current does not give rise to a period of slack water. 

 When the velocity of the rotary tidal current is least, it is known 

 as the minimum current, and when it is greatest it is known as the 

 maximum current. The minimum and maximum velocities of the 



North 

 k 



Midnight 

 12 



Noon 

 12 



^ Scale of Knots 



I I I I I I I I I I I I ^ 

 0.0 0.2 0.4 0.6 0.8 1.0 i.£ 



Figure 5.— Rotary current, Nantucket Shoals Lightship, forenoon of July 30, 1922. 



rotary current are thus related to each other in the same way as slack 

 and strength of the rectilinear current, a minimum velocity following 

 a maximum velocity by an interval of about 3 hours and being followed 

 in turn by another maximum after a further interval of 3 hours. 



Since the current day corresponds to the tidal day, it is convenient, 

 in determining the average hourly velocity and direction of the rotary 

 current, to make use of the times of high and low water at some 



