TIDAL CURRENTS O 



RELATION OF TIME OF CURRENT TO TIME OF TIDE 



In simple wave motioii the times of slack and strength of current 

 bear a constant and simple relation to the times of high and low 

 waters. In a progressive wave the time of slack water comes, theo- 

 retically, exactly midway between high and low water and the time 

 of strength at high and low water ; in a stationary wave slack comes at 

 the times of high and low water, while the strength of current comes 

 midway between high and low water. 



The progressive-wave movement and the stationary-wave move- 

 ment are the two principal types of tidal movements. A progressive 



Knots 



FiGTJBE 3. — Curves of daily and mixed types of reversing currents. 



wave is one whose crest advances, so that in any body of water that 

 sustains this type of tidal movement the times of high and low water 

 progress from one end to the other. A stationary wave is one that 

 oscillates about an axis, high water occurring over the whole area on 

 one side of this axis at the same instant that low water occurs over 

 the whole area on the other side of the axis. 



The tidal movements of coastal waters are rarely of simple wave 

 form; nevertheless, it is very convenient in the study of currents to 

 refer the times of current to the times of tide. And where the diurnal 

 inequality in the tide is small, as is the case on the Atlantic coast, the 

 relation between the time of current and the time of tide is very 

 nearly constant. This is brought out in figure 4, which represents 



429061°— 42 2 



