Historical Introduction 7 



At this point a discovery was made which for a time completely dis- 

 credited the notion that the winds are the cause of the ocean surface cur- 

 rents. Arago drew attention in 1836 to the results of a leveUng survey 

 across Florida which showed a difference of not more than 7^ in., and 

 probably less. This difference of level seemed much too small to produce 

 the Gulf Stream ; hence Arago advanced the idea that the cause of the 

 ocean currents is simply the density differences at the equator and poles 

 due to unequal solar heating. ' We should use the same theory for ocean 

 currents which we use to explain the Trade Winds', he stated. Seafarers 

 were still inclined to the theory of wind-driven currents. Our present ideas 

 of the magnitude of frictional forces in the Gulf Stream suggest, however, 

 that a head of 7 1 in. is adequate to drive the Stream. 



THE LAST HALF OF THE NINETEENTH CENTURY 



In 1844 an intensive study of ocean circulation was begun by Matthew 

 Fontaine Maury (1859), who was partial to Arago's theory. He dismissed 

 Rennell's theory of wind-driven circulation on the following grounds: 

 (i) assuming that the transport through the Florida Straits is the same as 

 that of the current off Cape Hatteras, he (Maury) deduced the depth at 

 Hatteras, completely ignoring the possibility that the Stream may mix 

 with its environment; (ii) since the depth deduced in this manner is less 

 than that at the Straits of Florida, Maury made the naiive claim that the 

 Gulf Stream would have to run uphill, and this reduced the wind theory to 

 absurdity. He did not realize that the horizontal pressure gradient along 

 the axis of the Stream is not controlled by the slope of the bottom boundary, 

 so that if the top surface slopes down toward the north, the current can 

 very well flow northward despite its diminishing depth. Maury was very 

 much confused concerning fluid mechanics, even though that science was 

 being rapidly advanced in his time ; Stokes's famous paper on the flow of 

 viscous fluids was pubhshed in 1845, and the significant work by CorioHs 

 had appeared in 1835. Maury uses such examples as the accumulation of 

 sawdust in the center of a basin filled \Adth water to explain the accumu- 

 lation of sargassum in the Sargasso Sea, and, to illustrate the Coriolis 

 force, the fact ( ? ) that railroad trains usually run off the rails to the right — 

 examples having no more than extremely doubtful applicabihty to the 

 ocean. He goes on to ask why, if the Atlantic circulation is a completely 

 closed circuit, there should be a piling up at any one place. This shows that 

 Maury did not understand Rennell's distinction between drift currents and 

 stream currents. There seems to be Uttle point in enumerating Maury's 

 misconceptions about the effect of the earth's rotation on ocean currents, 

 his roof-shaped current theory, his strange idea that waters of different 



