47 
(*the direction of the wind current at the very surface is assumed to 
be 45° to the right of the wind). Angles a and a’ are the differences 
between the direction of the surface current and that prevailing at 
the given fractions of the frictional depth. The same results were’ 
first shown graphically by Ekman in the form of a diagram, a copy 
of which is shown herewith. As an example of the use of Table 
VIII, and as further illustrated by Figure 23, if the frictional depth 
be 50 meters, then at a depth of 10 meters the water particles will 
flow in a direction 36° to the right of the current on the surface. 
So it is seen that if the 
surface velocity and the Y 
frictional depth be known, x 
the velocity and the di- 
rection of the pure drift 
current throughout the 
vertical range may be de- 
termined. Ekman has 
found that for practical 
purposes the equation 
may be simplified to 
Sevisition 0 et -) 
(j) is based upon the value 
of g equal to 1.025; and 
W represents the wind 
velocity in meters per 
second. It is easy to see 
from (j) that the greater 
the wind velocity the fs. 23.—Ekaman’s diagram in which the position and length 
‘f relatively of the successive arrows represent the direction 
deeper downward in an and velocity, respectively, of the pure drift current, down 
ocean will its effect pene- eeu, | ‘depth, set up as a result of wind and earth 
trate. Also since sin ¢ is 
zero at the Equator and 1 at the pole, it follows that given winds 
will exert a maximum influence at the Equator and the least effect 
at the pole. The density of the water is of some importance; a given 
wind current will be stronger and penetrate to a greater depth in a 
region of light water than in a region of heavy water. Table IX gives 
TasLe IX 
Wind velocity Latitude 
(Chita [REE 6 5° | 10° | 20° | 30° | 40° | 50° | 60° | 70° | 80° | 90° 
