60 MANUAL OF CURRENT OBSERVATIONS 
the nontidal current by adding it algebraically to the products. The results will be 
the corrected velocities with the sign in each case indicating the direction of flow. 
(See fig. 29.) 
158. Direction of current.—When obtaining the mean direction of the current for 
flood or ebb strength, care is to be taken not to include in the calculation an individual 
value that differs by an excessive amount from the apparent general average. Such 
a value should be encircled and omitted from the sum. Individual directions approxi- 
mating north should be made comparable by the addition or rejection of 360° when 
necessary. 
Rotary Currents 
159. The plotting of the original observed velocities, a procedure preliminary to 
the tabulation of reversing currents, is usually omitted when tabulating rotary currents. 
If such plottimgs are desired for checking purposes, it is recommended that all velocities - 
be plotted as positive ordinates regardless of the direction of the current with a separate 
plotting for directions when needed. As rotary currents are usually weak and irregular, 
the most satisfactory preliminary treatment is to resolve the observed velocities into 
north and east components. Reductions may, however, be carried on without such 
resolution if the observed periodic changes are fairly regular. Rotary currents with 
the movement principally along a single axis may be treated as reversing currents. 
160. Resolution of velocities ——The resolutions are usually made along a north- 
and-south and an east-and-west axis, the component velocities in the north and the 
east directions being taken as positive and those in the opposite directions as negative. 
The resolved values are designated as north and east components, and in order to avoid 
negative readings in the tabulations, a constant (usually 3 knots) is added to all resolved 
velocities. The resolution may be accomplished by multiplying each observed velocity 
by the cosine of its direction to obtain the north component and by the sine of its 
direction to obtain its east component. Resolutions may be made to accord with either 
true or magnetic north. 
161. The work of resolving the current velocities is facilitated by the use of Table 
6 which was specially prepared for the purpose. This table contains the north and 
east components of each tenth of a knot velocity from 0.1 knot to 3.0 knots inclusive 
for each 5 degrees of direction from 0 degrees to 355 degrees. To avoid negative values 
a constant of 3.0 knots is included in each component velocity given in_the table. 
Each group of four figures represents a north and an east component each expressed 
in knots and tenths, the first two figures being the north component and the last two 
the east component. Decimal points are omitted in the body of the table. Thus, the 
north and east components of a velocity of 0.5 knot setting 25 degrees are found, on 
the line marked ‘0.5’ and in the column headed ‘25°’, to be 3.5 knots and 3.2 knots 
respectively. If the direction of the velocity to be resolved is not a multiple of 5 
degrees the nearest direction that is such a multiple should be used in entering the table. 
Taking the directions to the nearest 5 degrees for this purpose will not affect the accuracy 
of results which are averages of a number of observations. For the hourly observations 
taken at lightships, the resolved velocities are entered directly in the columns provided 
in Form 270a which is used for recording these observations. In other cases, special 
provisions must be made for recording the resolved values. 
