77G 



THE NATIONAL GEOGRAPHIC MAGAZINE 



sary to ascertain its direction and A^elocity 

 not only on the surface, but below. The 

 surface current is the one affecting the 

 ship on its course, but it is also the most 

 affected by changes in the local wind, 

 which may be only temporary, while the 

 vast body of flowing water maintains its 

 direction, but feebly influenced by the 

 slight surface changes. 



An instrument was therefore designed 

 by which the lower currents could be 

 measured as well as the surface. It con- 

 sisted of a rudder free to take the di- 

 rection of the current, a compass needle 

 which of course pointed to the north, and 

 an apparatus for registering the flow. 

 Upon hoisting the instrument all of these 

 were locked at once, and upon its reach- 

 ing the surface the angle between the 

 rudder and the compass gave the direc- 

 tion, and the reading of the register 

 showed the velocity. 



Observations were usually made with 

 the current meter at depths of 3^ fath- 

 oms, 15, 30, 65, and 130 fathoms, and at 

 times to 150 and 200 fathoms, the in- 

 strument remaining at each depth during 

 a ])eriod of 30 minutes. 



The investigation began with these 

 ai)pliances in the narrowest parts 01 

 the Straits of Florida (between Fowey 

 Rocks, which is near Miami, and the Ba- 

 hamas) in order to find out the character- 

 istics of the stream at a point where it 

 would perhaps be least influenced by ab- 

 normal forces. .Vfter two seasons at 

 this point tlie research was extended to 

 the western part of the straits and to the 

 passage between Yucatan and Cuba to 

 gauge the water entering and leaving the 

 Gulf of Mexico. Afterward, the equa- 

 torial current and the flow into the Car- 

 ibbean between the islands were exam- 

 ined in order to compare what may be 

 called the source of the Gulf Stream with 

 the outlet as it leaves the Straits of Flor- 

 ida for the Atlantic. The stream oft' 

 Cape Hatteras, the flow outside the Ba- 

 hamas, and also south of Nantucket were 

 all examined. 



Some of the results of the investigation 

 were surprising. The volume of the 

 stream in i hour's flow has been men- 

 tioned, but the most valuable discovery 

 was that it changes its velocity daily and 

 monthly, and that predictions can be 



made as to the times of these variations. 

 As the tides change in height on the 

 coast, so the current changes in velocit}- 

 in the ocean. The equatorial current 

 along the South American coast runs 

 fastest about 6 hours before the moon 

 crosses the upper meridian ; between 

 Cuba and Yucatan the maximum is 10 

 hours before, and in the Straits of Flor- 

 ida off Fowey Rocks it is 9 hours. 



These variations at certain times in the 

 month amount to more than 3 miles per 

 hour in some parts of the stream and at 

 other times in the month may be less 

 than I mile per hour. 



During the month there is another 

 change taking place following the changes 

 in the declination of the moon north and 

 south of the equator. Two or three days 

 after the moon has passed the equator 

 the maximum velocity of the stream is 

 nearest the middle, but which it never 

 reaches ; it is always to the left of the 

 middle, and two or three days after the 

 moon's highest declination the maximum 

 IS well toward the left-hand edge of the 

 stream. 



Accompan3'ing these changes the tem- 

 perature of the stream fluctuates. At one 

 time during the month, also varying with 

 the declination of the moon, the direction 

 of the currents, particularly the lower 

 ones, incline slightly toward the axis of 

 greatest velocity, while at other times 

 they run more nearly parallel. This 

 causes an intermingling of the warm sur- 

 face Avith the colder water of the lower 

 strata. 



WII.VT CAUSKS the; GUI.F STREAM ? 



The various theories as to what causes 

 the Gulf Stream have been given, but it 

 seems that while Franklin's ideas are 

 nearest correct, they are not complete. 

 In the tropical regions there is a steady 

 movement of the air from east to west 

 known as the trade winds. South of a 

 certain line situated near the equator 

 these winds flow from a southwesterh^ 

 direction, while north of the equator 

 thev come from a more northeasterl\' 

 direction. The winds are not always 

 strong, nor are they constant in direction, 

 but they do not xavy much, and then 

 only for brief periods. 



