22 U. S. COAST AND GEODETIC SURVEY 



Greenwich transit, given in the table. Daily predictions of high and 

 low waters for the tide stations listed are included in the annual Coast 

 and Geodetic Survey tide tables. 



The harmonic constants derived from series of current observations 

 at a number of the lightship stations are given in tables 7, 8, and 9. 

 These constants consist of the amplitudes and the phase lags or epochs 

 of the more important periodic constituents of the current. Such 

 constants form the basis for daily predictions of the current. From 

 them also may be determined the general characteristics of the current 

 movement and various nonharmonic quantities which are usually 

 obtained directly from observations. 



In table 7 the north and east components of the movement are 

 represented separately, south and west being negatives of these. The 

 epochs have reference to the maximum velocities of the constituents in 

 a north or east direction. From the north and east component move- 

 ments, the rotational features of the constituents may be developed. 

 For details of such development, reference is made to United States 

 Coast and Geodetic Survey Special Publication No. 215, "Manual of 

 Current Observations." In tables 8 and 9 the constants represent a 

 reversing condition, the movement in the flood direction being positive 

 and that in the ebb direction, negative. The phase lags or epochs in 

 this case refer to the maximum flood of each constituent. 



Comparing the current harmonic constants for the lightship stations 

 with tidal harmonic constants derived from tide observations at 

 various points along the coast, it is found that the diurnal solar 

 constituent Si for the current is, relative to the other harmonic con- 

 stitutents, many times larger than for the tide. This relatively large 

 magnitude of the Sj current constituent was apparently unnoticed 

 until a few years ago when harmonic analyses of long series of offshore 

 current observations were begun. An investigation led to the con- 

 clusion that offshore the Sj current was due to a periodic land and sea 

 breeze rather than to the tidal forces. 



Although it appears generally too weak to be of much practical 

 importance, the Si current movement has been developed in detail for 

 a number of current stations largely because of the interest attached 

 to investigating a newly found or unusual phenomenon. The results 

 of the development appear in table 10 in the form of a velocity and a 

 direction for each hour of the solar day for each station. These 

 values were derived from the Si constituents of the north and east 

 component velocities as given in table 7. They were obtained by a 

 graphic process and an occasional value may differ slightly from the 

 corresponding value as derived mathematically. 



In table 10 the hours of the solar day are given in 75th meridian 

 time; the directions are in true degrees reckoned clockwise from north, 

 and the velocities in knots are given to three decimals. An examina- 

 tion of the tabular values reveals that as with other currents, the 

 characteristics of the Sj current differ from place to place. At most 

 stations it is largely rotary in character, but at a few, notably at 

 Nantucket Shoals Lightship, it is mainly reversing. The rotation is 

 clockwise at all the stations except Diamond Shoal Lightship, where 

 it is counterclockwise. The velocity at strength varies from about 

 0.02 knot at Boston Lightship to about 0.10 knot at St. Johns Light- 

 ship. In general, the Si current sets toward the land at about 15 

 hours or 3 p. m. and seaward, about 3 a. m. At a few locations, how- 



