REDUCTION OF CURRENT RECORDS 65 
be desirable to make all references to a single phase of the tide if other phases are not 
well defined. As with the reversing currents, predicted tides are in general to be 
preferred to observations for the purpose of reference. 
165. Observations taken at hourly intervals are to be referred to the nearest whole 
tidal hour, but when observations have been taken at half-hourly intervals it is often 
better to use half-hourly groups with reference to the nearest half hour. Currents 
observed at lightships are usually of the rotary type. Most of the current records 
reduced by the rotary method are obtained at lightships. After the reference station 
has been selected and the form prepared with the desired group headings, the usual 
procedure is first to indicate in the original record by colored pencil the times of the 
high and low waters to which references are to be made. With these times as guides, 
the assignment of each observation to its proper group is readily accomplished. Each 
observation should be tabulated once. When the same observation fulfills the require- 
ments of two gtoups, it may be included in either group but not in both. An occa- 
sional observation not belonging to any of the planned groups should be included in 
the nearest planned group. If such observations are numerous a rearrangement of 
the groups may be helpful. 
166. Reduction of rotary currents.—After the observations have been tabulated 
as described in the preceding paragraph, the hourly groups are summed and averaged. 
For series of observations extending over a number of months, it is recommended that 
the tabulations and reductions be made by calendar months, separate sums and aver- 
ages being obtained for each month. As these averages include both tidal and non- 
tidal current, the next step is to separate them from each other. When the original 
observations have been resolved into north and east component velocities, the corre- 
sponding components of the nontidal current can be obtained by averaging separately 
the resolved values for each month or for the period of the observations. For observa- 
tions taken at lightships, the daily sums may be conveniently entered in Form 270a 
at the bottom of the page, these sums being afterward brought together to obtain the 
monthly sums and averages. The velocity and direction of the resultant nontidal cur- 
rent may be obtained from its components by means of Table 7, after rejecting any 
constant that may have been introduced in the original resolution. 
167. The north and east components of the nontidal current are then subtracted 
algebraically from the corresponding components of the observed current as obtained 
for each of the hourly groups. The subtraction automatically eliminates any constant 
introduced in the original resolution since this constant appears in the component aver- 
ages for both the observed and the nontidal current. The velocity and direction of the 
resultant tidal current for each of the hourly groups may then be obtained from their 
components by the method already described. If the observations cover a period of 
less: than 1 month, the reduction factor described in paragraph 155 should be applied 
to the velocities of the tidal current. 
168. Graphic representation of rotary current.—The hourly velocities and direc- 
tions of the rotary current may be represented graphically either by rectilinear or 
polar coordinates. For rectilimear coordinates the cross-section paper printed from 
plate 4145 is recommended. Separate graphs with the same time scale are made for 
velocity and direction. The smallest subdivision of the horizontal ruling is usually 
taken to represent 0.05 knot in the velocity scale and 5° of azimuth in the direction 
scale. For the time scale the smallest subdivision of the vertical ruling is taken as 
the equivalent of 0.2 hour. Along this scale are indicated the times of the tidal phases 
to which the currents are referred. To promote uniformity in this work it is suggested 
