256 



an established reference current station, are ascertained and averaged. 

 If the tides and currents have much diurnal inequality the intervals 

 of the greater flood and ebb strengths preferably are referred to the 

 times of higher high and lower low water. By adding the established 

 intervals between a lunar transit at Greenwich and the times of the 

 tide or current phases at the reference station, the Greenwich intervals 

 at the given station are then determined. These Greenwich intervals 

 are preferred to lunitidal intervals reckoned from the time of a lunar 

 transit at the station, because the difference between the Greenwich 

 intervals at any two stations gives the difference between the respective 

 times of their currents directly, without any correction for the differ- 

 ence in the longitudes of the stations. If the current had a simple 

 harmonic fluctuation with the speed of the M2 component, the dura- 

 tion of each increase in velocity from slack to strength, and of each 

 decrease from strength to slack, would be one-quarter of the semilunar 

 day of 12.42 mean solar hours. To establish a single time interval 

 for all four slacks and strengths at a station, the "mean current hour" 

 at the station is computed by averaging the Greenwich intervals of 

 the strength of the flood, the slack before flood increased by 3.10 

 hours, slack after flood decreased by 3.10 hours, and strength of flood 

 increased or decreased by 6.21 hours, after bringing all of these sums 

 into the same semilunar day by adding or subtracting 12.42 hours 

 as may be necessary. 



501. In estuaries and tidal rivers the fresh-water flow may be so 

 great that the current remains in one direction and the velocity varies 

 from a maximum to a minimum without passing through slack. 

 Again, the overcurrents at some stations are so large that the current 

 reaches two maximum velocities during each flood, or ebb, or both. 

 The direction of the current may even reverse between these maxima. 

 The measures taken in these special cases need not be elaborated here. 



502. Reduction of average current strengths. — Since the tidal currents 

 in estuaries and other closed channels, and in inlets to a closed basin, 

 are due to the filling and emptying of the tidal prism of the channel 

 or basin, the successive strengths of the tidal flood and ebb at a current 

 station in the channel are nearly proportional to the concurrent 

 ranges of the tide at a repiesentative tide station on the waterway. 

 The average tidal flood and ebb strengths, determined from a short 

 series of observations, therefore may be converted into long-term 

 averages by multiplying them by the ratio of the established mean 

 tidal range at the tidal station to the average observed range during 

 the period of the current observations. Obviously, this correction 

 is not to be applied to any constant component of the current which 

 may be produced by fresh-water outflow, or other cause, during the 

 period of the observations. 



