8 MANUAL OF CURRENT OBSERVATIONS 
the velocity of the flood current, while near the bottom of the stream the tidal current 
may be little affected by this fresh water discharge. This tends to increase the differ- 
ence between the surface velocity and that at the bottom when the current is ebbing, 
but during the flood period the surface velocity may become less than that at the bottom 
of the stream.. The fresh water discharge also affects the surface currents by advancing 
the time of slack before ebb and retarding the time of slack before flood. 
Rotary Tidal Currents 
19. Rotary currents are those that flow continuously with the direction of flow 
changing through all points of the compass during the tidal cycle. Rotary currents 
are usually found offshore where the direction of flow is not restricted by any barriers. 
The tendency for the rotation in direction has its origin in the deflecting force of the 
earth’s rotation, and unless modified by local conditions the change of direction is 
clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemi- 
sphere. The velocity of the current usually varies throughout the tidal cycle, passing 
through two maxima in approximately opposite directions and two minima with the 
direction of the current at approximately 90° from the direction of maximum velocity. 
The maximum velocity attained on a rising tide or near high water corresponds to the 
flood of the reversing current, and the maximum velocity attained on a falling tide or 
near the time of low water corresponds to the ebb of the reversing current. 
North L+3 
H+3 
! | knot | 
FicuRE 10.—Mean current curve for Nantucket Shoals Lightship, referred to tides at Boston. 
