253 



moon and sun. The currents at any station may therefore be resolved 

 into harmonic components of constant amphtudes, whose speeds 

 are the same as the speeds of the tidal components. The mean 

 amplitudes and the epochs of the several current components at the 

 selected reference stations are determined from an harmonic analysis 

 of the actual current velocities and directions at the station, measured 

 by float or current meter, at hourly or half-hourly intervals for a:, 

 sufficient number of days. The dials of a tide-predicting macliine are; 

 set at the component current amplitudes reduced to the current year, 

 and at the component phases at the beginning of the year; and the 

 current predictions at the reference stations are run off like the pre- 

 dictions of the tides. At stations where the tide is of the rotary 

 type, the harmonic constants of the east-west and north-south com- 

 ponents of the tide may be similarly computed, their resultants in 

 the prevailing direction of the maximum and minimum currents 

 ascertained, and the predicted times and strengths of the currents in 

 these directions run-off from the macliine. 



494. The corrections to be applied to the predicted times and 

 strengths of the current at a designated reference station to obtain 

 those at a secondary station are derived from the average intervals 

 between a lunar transit and the times of slacks and strengths at the 

 two stations, and the average tidal current velocities at the strengths 

 of the current. The compilation of this data is termed the non- 

 harmonic reduction of the observations, as distinguished from the 

 harmonic reduction by which the harmonic constants at the reference 

 stations are obtained. 



495. Accuracy of tidal current predictions. — The actual times of 

 slack or strength of the current at a station occasionally differ by as 

 much as half an hour from the predicted times, and in rare instances 

 by as much as an hour. Comparisons of the predicted and observed 

 times show that more than 90 percent of the slack waters have been 

 within half an hour of the predicted. Both the times and the strengths 

 of the currents in tidal estuaries may be greatly altered by unpredict- 

 able variations in the fresh-water discharge, and in inlets and straits 

 by the storm tides and lesser variations due to winds and other meteor- 

 ological disturbances. 



496. Methods employed for current observation and reduction. — The 

 procedure adopted by the Coast and Geodetic Survey in taking, re- 

 cording, and reducing current observations is set forth in detail in the 

 Manual of Current Observations (Special Publication No. 215, U. S. 

 Coast and Geodetic Survey). The harmonic; reduction and predic- 

 tion of tides has been explained in chapter II. The harmonic con- 

 stants of the tides, besides providing the means for tidal predictions, 

 afford an understanding of the variations in the tide, and of the tidal 

 datum planes to wliich works in tidal -w^aters are referred. Because 



