NATURE OF THE STREAM 133 



Side by side with the steady compihng of data for charts went 

 the growth of physical theory. The logical though partial 

 explanation offered by Benjamin Franklin will serve as a start- 

 ing point from which to trace its development into the con- 

 cepts of present-day science. 



Franklin's wind-current theory was both supported and 

 extended by Rennell. In his An Investigation of the Subject 

 of the Currents of the Atlantic Ocean he distinguished clearly 

 between drift currents produced by the force of winds acting 

 directly on waves, and stream currents which flow as a result 

 of a difference in water level. According to Rennell, the trade 

 winds are responsible for the equatorial currents, which in 

 turn heap water in the Caribbean Sea and the Gulf of Mexico 

 above the general ocean level. The increased height of this 

 water causes a stream current to flow, and this is the Gulf 

 Stream. Since this theory requires the level of these seas to be 

 higher than the general sea level, it became discredited when 

 Arago insisted that they are actually lower. In Poggendorf's 

 Annalen der Physik und Chemie, published in 1836, he 

 pointed out that surveying parties between Chagres and 

 Panama on the opposite sides of the Isthmus of Panama had 

 found the sea level of the Atlantic Ocean to be from three to 

 five feet lower than the Pacific. Later these measurements 

 were proved incorrect, but for some time to come this was a 

 serious setback for the wind-current theory. 



Arago, with a rival hypothesis, started a battle of opinion 

 that raged throughout the nineteenth century. He thought 

 that the heating of tropical sea water causes it to flow outward 

 from the equator toward the cooler regions round the poles, 

 just as the unequal heating of the air causes the great wind 

 systems of the earth. He also brought in the effect of the 

 earth's rotation to explain why the currents do not run simply 

 from south to north: ''. . . the rotation of the earth ought 

 principally to be taken into view, and that this, together with 



