COASTAL CURRENTS, ATLANTIC COAST 17 



mile that the time the pole is allowed to drift bears to an hour. By 

 this means the velocity in knots (nautical miles per hour) and tenths 

 is read directly from the current line. The direction toward which the 

 pole drifts is observed by means of a pelorus which is used in conjunc- 

 tion with the ship's compass. The pelorus used is a metal disk about 

 8 inches in diameter and graduated clockwise for every 5 or 10 degrees. 

 The current pole used on the lightships was 15 feet long and was so 

 weighted as to float with 1 foot of its length extending above the water 

 surface. The current measured was therefore the average current for 

 the first 14 feet of depth. In most cases the current observations were 

 taken hourly throughout the 24 hours of each day. The velocity and 

 direction of the wind at the time of each current observation were 

 noted and recorded. The velocity of the wind was usually estimated 

 but in some cases was measured by anemometer. The wind direction 

 was determined by the ship's compass. 



METHODS OF REDUCING THE OBSERVATIONS 



PRELIMINARY STATEMENTS 



As the current movement to be studied was a combination of a 

 number of constituent movements, it was necessary to identify and 

 insofar as possible to isolate and evaluate the more important of these 

 constituent currents. For this purpose a number of reduction proc- 

 esses were used by means of which three main classes of results repre- 

 senting three important elements of the current movement have been 

 derived. These three groups of results are presented in this volume 

 under the headings: tidal currents, non tidal currents, and wind 

 currents. 



Because of their periodicity, the tidal currents can be readily separ- 

 ated from the observed movement, their characteristics determined, 

 and their more important harmonic constituents calculated with con- 

 siderable precision, provided a long series of observations has been 

 secured. 



The nonperiodic constituents of the current cannot readily be sep- 

 arated from each other. The values given under the heading "non tidal 

 currents" are the residual currents for the periods and stations indi- 

 cated. They include wind effects in combination with other nontidal 

 effects such as oceanic circulation and drainage. 



The wind currents given are the average currents that accompanied 

 the indicated wind directions and velocities at the stations named 

 during the periods stated. They are therefore not in all cases purely 

 the result of winds but may include other nontidal effects. 



The methods of reducing the reversing and rotary types of tidal 

 currents as well as nontidal currents and wind currents are discussed 

 in detail in U. S. Coast and Geodetic Survey Special Publication No. 

 215, "Manual of Current Observations." They will be described here 

 briefly, much of the detail that is not essential to an understanding 

 of the results being omitted. 



NONHARMONIC REDUCTION OF REVERSING TIDAL CURRENTS 



Under this heading is briefly described the process which with some 

 modifications in individual cases was used for reducing tidal currents 

 having relatively large velocities — usually half a knot or more — and 



