52 MANUAL OF CURRENT OBSERVATIONS 
observing vessel, the sextant angle being added if measured to the right of the reference 
object and subtracted if measured to the left. 
133. The direction of subsurface currents as indicated by the Ekman current 
meter may for convenience be recorded directly in Form 270 m the column provided 
for direction of pole, but such entries must be suitably marked with a reference to an - 
explanatory note. A correction for compass variation will be required to obtain the 
true azimuth of the current, but corrections for deviation are usually disregarded as 
the instrument itself is constructed mainly of nonmagnetic substances and there are 
practical difficulties in determining with any degree of accuracy the effect of the proxim- 
ity of the observing vessel. 
Reversing Currents 
134. For a discussion of reversing currents see paragraphs 6-18. In the non- 
harmonic reduction of these currents, the results usually sought are (a) times of strength 
of flood, strength of ebb, and slack waters as referred to the moon’s transit; (6) mean 
velocity of current at times of strength of flood and strength of ebb; (¢) mean azimuth 
of current at times of strength of flood and strength of ebb; and (d) velocity and azimuth 
of nontidal current. Besides referring the times of the current phases to the moon’s 
transit as a standard reference for all places, comparisons are also made, with tide or 
current phases at certain selected primary stations. This may be done both as an 
intermediate step in the reduction and to obtain differences which can be conveniently 
used with regular predictions for the primary station. On the Atlantic coast of the 
United States where the diurnal inequality in the tides is relatively small, the reductions 
are usually carried on without any distinction between morning and afternoon tidal 
cycles; but on the Pacific coast where the diurnal inequality is relatively large, a dis- 
tinction is usually made if there are sufficient observations to develop this mequality. 
Following are described the several steps in the nonharmonic reduction of reversing 
currents. 
135. Plotting reversing currents.—Observed velocities are plotted on cross-section 
paper with times as abscissas and the velocities as ordinates. Cross-section paper 
printed from plate 3391, with the 24 hours of the day at the top, is recommended for 
the purpose. The velocity scale usually adopted is one knot for each principal division 
(about 0.6 inch) of the form, with each subdivision representing one-tenth of a knot. 
Other velocity scales may be used, however, when more suitable. 
136. A horizontal line representing zero velocity is to be ruled across the form 
with black ink. Flood velocities are to be plotted above this line as positive ordinates 
and ebb velocities below the line as negative ordinates, using ink for this plotting. If 
the direction of any observed current differs greatly from the normal flood and ebb 
movement, the plotted velocity point should be encircled to indicate a lack of definite- 
ness. Several days of record may often be plotted on a single sheet if the zero velocity 
line for successive days is lowered by an amount sufficient to keep the plottings separate. 
When the velocities have been measured by pole near the surface and by meter at 
several subsurface depths, it is frequently advantageous to use a sheet for each day, 
plotting the pole velocities near the top of the sheet and the meter velocities below in 
order of depth. 
137. Curves are now drawn with pencil to follow as near as practicable the general 
trend of the plotted velocity points. A single point standing apart from the general 
