48 



thefrictional 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 (&) 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 (h) 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 surfaoe 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 %\'inds, 

 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 " Earth Rota- 

 tion and Ocean Currents," Arkiv for Matematik, Astronomi och 



