48 
the frictional depth for different wind velocities at different latitudes and 
is computed by means of equation (j). The table is also based upon 
several assumptions, two important ones of which are, (1) the water 
‘mass be homogeneous, and (2) the depth to the bottom be great com- 
pared with the frictional depth. Such restrictions, as a matter of 
fact, differ from conditions actually met in the ocean, but the table 
gives an idea, nevertheless, of the depth of the pure wind current 
in the open sea, bearing in mind that when great variations in the 
density are found, the frictional depths will be less. At such places 
where abrupt transitions in the density of a water column takes 
place (e. g., a well-pronounced condition during the summer when 
the superficial layers become relatively light), then the boundary 
between the surface water and the heavier underlying mass acts as 
a virtual bottom in determining the development of the pure wind 
current. The density curves for the Grand Banks column, for ex- 
ample, often reveal an abrupt transition at a depth of about 20 to 
30 meters, and such a discontinuity layer probably indicates the 
lower limit of the drift current at the time. If the depth of the upper 
layer be less than the frictional depth, as found by Table IX, then the 
effect of earth rotation is small and the direction of the wind current 
will more or less parallel the direction of the wind. On the other 
hand, at those places where homogeneous water is found extended 
downward 200 or 300 meters (e. g., in an ocean which has been 
subjected to the effect of an entire winter’s cooling), then we may 
expect wind currents (after a day or so outside of the Tropics) to 
develop in characteristic form. (See fig. 23. p. 47.) 
A distinction has been made between (a) the direct frictional 
effect. of the wind blowing over a surface in the deep open sea, as it 
‘sets in motion a pure wind current, and (b) a similar effect of the 
wind but under the influence of coast lines or other hinderances, by 
which the water becomes amassed against (or sucked out from) such 
boundary surfaces in the sea. Winds classified as (b) bring to the 
problem a consideration of two subsequent movements known as 
“the mid-water current” and the “bottom current.’’ The bottom 
current compensation for the surface current as the latter flows 
toward (or away from) boundaries. As an example of the building 
up ability of far reaching currents by a system of prevailing winds, 
we might regard the northwestern North Atlantic region along the 
North American coast, stretching from Baffin Land to the southern 
reaches of Newfoundland. Normal atmospheric distribution, espe- 
cially well marked in this region during the December—March period, 
furnishes a strong northwesterly wind component, which a glance at 
the map will show lies approximately parallel to the coastal trend, 
Baffin Bay to Cape Race. Ekman has pointed out (see “ Karth Rota- 
tion and Ocean Currents,’’ Arkiv fér Matematik, Astronomi och 
