REDUCTION OF CURRENT RECORDS 53 
trend of all the points, or a point that has been encircled to indicate some uncertainty 
in the observation, may be disregarded when drawing the curves. With the above 
exceptions, the departure of the curve from the plotted points of observed velocities 
should in general not exceed 0.2 knot by the scale that is used. In general, it is desirable 
to have the curve pass through the points of observed maximum flood and ebb velocities 
unless these are clearly erroneous. 
138. In some cases it is necessary or desirable to depart somewhat from the pro- 
cedure outlined above for drawing the velocity curves. Plottings for some localities 
show such roughness, due to surge or other conditions, that a curve following the general 
trend will unavoidably depart more than 0.2 knot from many of the plotted velocity 
points including the points of largest flood and ebb velocities. Many plottings form 
curves smooth enough for the purpose at hand when the points are connected succes- 
sively by straight lines. This method may be used for all reversing-current velocity 
curves; a smooth curve being drawn if it is needed for preparing the tabulations 
described in succeeding paragraphs or for other purposes. 
139. When sub-surface velocities are observed with a meter that does not record 
the direction of the current, it is sometimes possible to infer the direction from surface 
observations with current pole, especially in passageways where the current floods and 
ebbs with considerable regularity. In this case the velocities may be plotted in the 
manner already described, with the flood above the zero line and the ebb below it. 
If, however, there is considerable uncertainty in the direction of the sub-surface cur- 
rents, it is best to plot all velocities above the zero line, noting the fact on the plotting. 
140. In addition to the velocity curves it usually is desirable to include on the 
plotting sheet the current directions observed at the times the velocities were measured 
and a notation of wind conditions during the observations. The current directions in 
degrees are tabulated m a horizontal column above the velocity curve each direction 
on the same vertical line as the corresponding velocity point; or if desired the directions 
may be plotted below the velocity curve using the same horizontal time scale as for 
the velocity plotting, and a suitable vertical direction scale. The wind note should 
state briefly the general direction and velocity of the wind and indicate the times of 
changes in direction or velocity. 
141. Tabulation of strengths and slacks.—Form 451 (fig. 28) is used for this 
tabulation. The times of flood and ebb strengths and slack waters are taken from 
plotted curves and entered in the form to the nearest tenth of an hour, the 24-hour 
system being used in recording the hour of the day. Attention is to be given to the 
general trend of the curve when selecting the time of the strength of current, the aim 
im general being to bisect the horizontal chord that represents a velocity about 0.1 
knot less than the maximum velocity as read on a smooth curve. The practice of 
picturing mentally a smoothed curve and selecting the time of strength from the curve 
thus pictured is satisfactory and labor saving when the curve formed by the plotted 
points is reasonably smooth. If there is considerable roughness a smooth curve should 
be drawn. 
142. The rule given below for selecting the time of a current strength from an un- 
smoothed curve is helpful, particularly if the selecting is done by an inexperienced 
worker. 
Mark with pencil near the zero-velocity line to the nearest tenth of an hour 
the following three times each of which is an approximation to the time of 
strength: 
(1) The time midway between the preceding and following slack waters. 
