REDUCTION OF CURRENT RECORDS 83 
would indicate the square of the velocity. A special square-root reading scale for 
taking the velocities direct from the marigram may be prepared as follows: 
Let Y=distance of any velocity graduation from zero of scale. 
Then 
Y (in inches)=0.1 X (scale factor) < (gear ratio) X (velocity)?_____._-_---___ _ (24) 
With the scale factor and gear ratio each unity, 1 knot of velocity would be represented 
by 0.1 mch on the marigram, 2 knots by 0.4 inch, 3 knots by 0.9 inch, ete. With a 
seale thus constructed the velocity of the tidal current may be taken directly from the 
marigram. Any nontidal current which is to be included may afterwards be applied. 
Wind Reduction 
201. The term ‘“‘wind current” is applied to a flow of water resulting from the 
action of the wind. Wind currents usually occur m combination with the periodic 
tidal current as well as with the more or less permanent nontidal current maintained 
by the general circulation of the oceanic water or by local conditions such as the fresh 
water discharge from a river. From theoretical considerations, currents produced by 
the wind in the open sea in the northern hemisphere will set to the right of the direc- 
tion towards which the wind is blowing, and in the southern hemisphere to the left 
of this direction. The actual direction, however, may be largely affected by local con- 
ditions such as the underwater topography and the position of the shoreline. 
202. For wind reductions long series of observations are necessary in order to 
average out the effects arising from other causes. Observations covering a year or 
more taken from lightships anchored well offshore have generally afforded the best 
available data for the purpose. Preliminary to the reduction the hourly velocities 
are resolved into north and east components in accordance with the instructions in 
paragraphs 160-161. 
203. The hourly velocity components of the current are grouped according to the 
direction and velocity of the wind. Form 594a (fig. 35) is used for the purpose. The 
sixteen wind directions—north, north-northeast, northeast, ete.—are generally used. 
A separate sheet is provided for each wind direction, and in this sheet separate columns 
are used for different wind velocities according to certain specified limits. Observations 
taken when the wind is less than 5 miles per hour are usually excluded. The first group 
is taken to include all observations when the wind velocity is from 5 to 15 miles per 
hour with an assumed average velocity of 10 miles per hour. The second group includes 
wind velocities from 16 to 24 miles per hour inclusive with an assumed average velocity 
of 20 miles per hour. Similarly, provisions are made for higher average velocities in 
multiples of 10 miles per hour. For a series of observations covering a year, it will 
generally be necessary to use several pages of the form to include all observations 
pertaining to any single wind direction. In this case page sums are taken and brought 
together on a single summary sheet (fig. 36) upon which component means and cor- 
responding resultant velocities and directions for the several groups are also obtained. 
204. In Form 270a the wind directions often are on a different basis from that of 
the current directions used in determining the component velocities. For example, 
the wind directions may be compass readings and the current directions magnetic 
(compass readings corrected for deviation). In Form 594a the values for both wind 
