4 U. S. COAST AND GEODETIC SURVEY 



exhibited by tides. Stronger currents than usual come with the spring 

 tides of full and new moon and the weaker currents with the neap tides 

 of the moon's first and third quarters. Likewise, perigean tides are 

 accompanied by strong currents and apogean tides by relatively 

 weaker currents; and when the moon has considerable declination, 

 the currents, like the tides, are characterized by diurnal inequality. 



As related to the moon's changing phases, the variation in the 

 strength of the current from day to day is approximately proportional 

 to the corresponding change in the range of the tide. The moon's 

 changing distance likewise brings about a change in the velocity of 

 the strength of the current which is approximately proportional to 

 the corresponding change in the range of the tide; but in regard to 

 the moon's changing declination, tide and current do not respond 

 alike, the diurnal variation in the tide at any place being generally 

 greater than the diurnal variation in the current. 



The relations subsisting between the changes in the velocity of the 

 current at any given place and the range of the tide at that place 

 may be derived from general considerations of a theoretical nature. 

 Variations in the current that involve semidiurnal constituents will 

 approximate corresponding changes in the range of the tide; but for 

 variations involving diurnal constituents the variation in the current 

 is about half that in the tide. 



TYPES OF REVERSING CURRENTS 



Since tides and tidal currents are merely different . aspects of the 

 tidal movement of the waters, the former being the vertical movement 

 and the latter the horizontal movement, it is to be expected that tidal 

 currents would show different types, corresponding to the different 

 types of tide. Observations prove this to be the case. Reversing 

 currents may be readily classed under the three types of semidaily, 

 daily, and mixed. The semidaily type is one in which two flood 

 strengths and two ebb strengths occur in a tidal day, with but little 

 inequality between morning and afternoon currents. Figure 1, illus- 

 trating the current in The Narrows, New York Harbor, may be taken 

 as representative of this type. 



The daily type of tidal current is characterized by one flood and 

 one ebb in a day. The upper diagram of figure 3, which represents 

 the current as observed in the entrance to Mobile Bay, Ala., on May 

 2-3, 1918, exemplifies this type of current. The mixed type of tidal 

 current exhibits two floods and two ebbs in a day with considerable 

 inequality between the forenoon and afternoon cycles. The lower 

 diagram of figure 3, which represents the current observed in Rich 

 Passage, Puget Sound, Wash., on March 29-30, 1917, illustrates this 

 type of current. 



In general, it may be said that with reversing currents a given type 

 of current accompanies a like type of tide; that is, semidaily currents 

 occur with semidaily tides, mixed currents with mixed tides, and daily 

 currents with daily tides. But as noted in considering the variations 

 in strength of current, the variations in the current that involve semi- 

 daily constituents will approximate corresponding changes in the 

 range of the tide, while in those involving daily constituents the varia- 

 tion in the current is about half that in the tide. Hence the diurnal 

 inequality in the current at any place is generally less than in the 

 tide at that place. 



