Ocean Currents 



14 



The winds of the North Atlantic 

 Ocean, as also of the several other 

 oceans — the South Atlantic, South Pa- 

 cific, North Pacific, and the Indian — 

 are governed mainly by the presence of 

 an almost permanent area of high ba- 

 rometer covering the main body of the 

 ocean, around which the winds con- 

 stantly circulate, the circulation in the 

 Northern Hemisphere being in the same 

 direction as the hands of a clock, 

 in the Southern Hemisphere in a con- 

 trary direction, or "with the sun" 

 in either hemisphere, as it is expressed 

 by sailors. In the North Atlantic the 

 center of this area lies somewhat to 

 the southwest of the Azores. On the 

 southern slope of this barometric pla- 

 teau the winds have an easterly direc- 

 tion — the northeast trades; on the north- 

 ern slope, a westerly. These west- 

 erly winds, however, exhibit none of 

 the constancy of the trades, being con- 

 stantly interrupted by the wind systems 

 proper to the alternate areas of high 

 and low barometer which move across 

 continent and ocean from west to east, 

 and which form the governing feature 

 of our own weather, the wind backing 

 to the southeast with falling pressure, 

 hauling to northwest with rising. Just 

 as in the case of the trades, only to a 

 much less extent; there is, however, a 

 sufficient easterly component remain- 

 ing to impart to the waters of the sea 

 below the surface a distinct easterly mo- 

 tion, while on the surface itself there 

 is apparently an utter lack of definite 

 direction other than the fact that the 

 direction of the current ordinarily agrees 

 with the direction of the wind. How 

 true this is may be gathered from a com- 

 parison of the observed winds and the 

 observed currents for a given area ; for 

 instance, the 5 square included between 

 the parallels 4o°-45° N., 3O -35° W — 

 about in mid-ocean. The total number 

 of wind observations recorded for the 

 square was 8,898; of reliable current 

 observations, 719. Dividing each of 



these up into quadrants and setting the 

 current under that wind quadrant to 

 which they are due, we have the fol- 

 lowing percentages: 



N. E. S. E. S. W. N. W. 



Winds 16 20 36 28 



3i 31 



Currents 20 



IS 



THE CONSTRUCTION OF CURRENT 

 CHARTS 



For our knowledge of the currents of 

 the sea as tabulated in the current 

 charts used by navigators — the move- 

 ments of the waters as they actually 

 take place — we are dependent upon 

 ships' observations. When at sea the 

 position of a vessel at noon of each day 

 is determined by two independent meth- 

 ods. The first of these is known as the 

 position by observation, and means, as 

 its name implies, the position of the ves- 

 sel as found by actual astronomical ob- 

 servation. The second is known as the 

 position by dead reckoning, and is the 

 position as found by reckoning up the 

 vessel's progress from noon of the pre- 

 vious day, the compass giving the direc- 

 tion, the log the speed. In a majority 

 of cases these two positions fail to agree. 

 The astronomical position is then as- 

 sumed to be correct, and the difference 

 between them is set down as the cur- 

 rent during the intervening 24 hours. 



Thus let A be the position by obser- 

 vation at noon of a given day, B' the 

 position by dead reckoning at noon of 

 the following day — i. e. , the position de- 



rived from a consideration of the course 

 and distance during the intervening 24 

 hours. Suppose, however, that astro- 

 nomical observations show that the 



