Ocean Currents 



37 



June 2o°-25° N. 



5o°-55° W. 



N. 



N. N. E. 

 N. E. 

 E. N. E. 

 E. 



E. S. E. 

 S. E. 

 S. S. E. 



Other squares show similar variations; 

 some greater, some less. 



THE IMPULSE COMMUNICATED BY THE 

 WINDS TO THE SURFACE WATER 



Let us now examine the effect of such 

 a system of winds in impelling through 

 surface friction the water with which 

 the}' come in contact. 



If through any cause a thin laj^er of 

 liquid is set in motion in its own plane 

 with a given velocity, the layer imme- 

 diately below it, and with which it is in 

 contact, does not remain at rest, but 

 likewise receives an impulse. This 

 second layer exercises a like impulse 

 over the third, the third over the fourth, 

 and so on, the velocity ultimately at- 

 tained by each successive layer being pro- 

 portional to its distance from the bottom 

 layer, which is supposed to be at rest. 

 In the case of sea water, the rapidity 

 with which this velocity is propagated 

 downward is very slight. It has been 

 calculated, for instance, that a period of 

 239 years would elapse before a layer at 

 a depth of 50 fathoms would attain a 

 velocity equal to half that at the sur- 

 face, and for a surface current of given 

 velocity to transmit its proper propor- 

 tion of that velocity to a depth of 2,000 

 fathoms would require an interval of 

 200,000 years, the surface current flow- 

 ing steadily all this time. Such surface 

 currents do not exist, nor do winds 

 capable of producing them. The trades, 

 as we have seen, fluctuate from day to 

 day and, indeed, from hour to hour, 

 and the surface currents fluctuate in 

 obedience to them. 



It has been stated, however, that the 

 fluctuations of the trades rarely carry 



them out of the eastern semicircle, and 

 that in point of fact 90 per cent of the 

 winds that blow in the region of the 

 trades do come from that semicircle. 

 There is thus always a westerly compo- 

 nent in the motion of the air, coupled 

 with a component which is sometimes 

 northerly, sometimes southerly. For 

 each alteration in the direction of the 

 wind there is a correspond ing alteration 

 in the direction of the surface current, 

 the new direction being the resultant of 

 the old direction and the direction which 

 would be imparted to it by the new wind 

 acting alone. These, however, affect 

 only the waters immediately at the sur- 

 face. Thus, to cite a specific example, 

 observations at the Adlergrund light- 

 ship, in the Baltic Sea, have shown that 

 while the water at the surface responds 

 almost immediately to a change in the 

 direction of the wind, the water at the 

 depth of 2^ fathoms does not feel its 

 effects until an interval of 24 hours has 

 elapsed. The steady westerly compo- 

 nent is then the only one felt in the 

 region of the trades at some little depth 

 below the surface, and this is sufficient 

 ■ to impart to "the entire body of water 

 occupying the equatorial regions of the 

 earth a westerly motion. 



It is of some interest to note the ve- 

 locity imparted to the surface water by 

 winds of a given force. A comparison 

 of a large number (658) of wind and 

 current observations in the equatorial 

 regions gave as the set imparted by a 

 wind of force 4 on the Beaufort scale, 

 corresponding to 20 miles per hour, a 

 current velocity of 15 miles per day. 

 The figures are taken from the ' ' Me- 

 teorological Data for Nine io° -squares 

 of the North Atlantic Ocean," pub- 

 lished by the Meteorological Committee 

 of the Royal Society. 



The system of surface currents pro- 

 duced by such a system of winds as the 

 trades has been experimentally studied, 

 using for this purpose a miniature ocean, 

 the surface of the water being lightly 



