and his Phoenician sailors realize that they were being helped by 

 the Agulhas Current, and later hindered by the Canaries Current, 

 in their much-disputed circumnavigation of Africa? By the thir- 

 teenth century, the Agulhas Current must have been known, for 

 Marco Polo had heard of it. With every improvement in the art of 

 navigation, and better means of determining position at sea, men 

 gained more and more knowledge about the surface currents, and 

 the discrepancies between observed and estimated positions — which 

 must have alarmed the sailor in unknown currents — could be 

 turned to his advantage or avoided. 



But it was a long time before this gradually accumulating know- 

 ledge was collected, put on charts, and made available to seafarers 

 in general. Thus in 1770 we find the Board of Customs in Boston 

 complaining to the Lords of the Treasury in London that the mail 

 packets usually took two weeks longer to cross the Atlantic than 

 did merchant ships. Benjamin Franklin, who at that time was Post- 

 master General, found that although many fishermen and merchant 

 ships' captains were familiar with the Gulf Stream, the mail packet 

 captains were not, and he had a chart of its course drawn up and 

 printed for their benefit. Matthew Fontaine Maury, of the United 

 States Navy, was the first to organize the systematic collection of 

 current observations. About the middle of the nineteenth century 

 he extracted from ships' logs the scattered observations of currents, 

 winds, and weather and produced charts of the average values. He 

 persuaded mariners to make better observations and rewarded 

 those who co-operated with free copies of his charts and sailing 

 directions. Their use by sailing ships shortened ocean passages by 

 several days, or even weeks, and his charts and methods of com- 

 piling them are the immediate forerunners of those in use today. 



At the same time as this increase in practical interest and use of 

 currents, there was much scientific speculation about the causes of 

 the great ocean current systems. Two rival hypotheses were the 

 source of much controversy. Benjamin Franklin's view, adhered to 

 by Rennell, James Croll and many others, was that the prevailing 

 winds, by their drag on the sea surface, were the cause of most of 

 the currents. Others, including Maury and Dr. W. B. Carpenter, 

 believed that differences of density in the water, caused by unequal 

 heating in different latitudes, provided the main driving force for 

 the currents. 



But the arguments put forward in support of these ideas were 

 merely qualitative, and inadequate to establish which process was 

 most important. Only in the last decade or two have quantitative 

 theories of the circulation of the ocean been devised. 



Our knowledge and charts of the surface currents of the oceans 

 have been built up gradually, over a period of several decades, 

 from many thousands of observations of drift experienced by ships. 

 When a chart of surface currents is made in this way, the average 

 currents stand out clearly. 



In general, the ocean currents form a pattern of nearly closed 

 loops which turn clockwise in the Northern Hemisphere and 

 counterclockwise in the Southern Hemisphere. These circulating 

 systems correspond roughly to the pattern of the prevailing winds. 

 Exactly how the winds drive the currents along is not so easy to 

 see, though. The most obvious effect of the wind, when it blows 

 over the sea surface, is to raise waves ; part of the energy transferred 



A scientist empties a water sampling 

 device aboard tlie Challenger during the 

 expedition of 1872-76. Tiiis particular 

 instrument was used for taking water 

 samples immediately above the sea floor. 



The currents and surface temperature | 



chart on the following pages " 



shows the direction of movement of surface 

 water over the world ocean. Winds, sea 

 surface currents, and sea surface temperatures 

 are all interrelated. Over much of 

 the ocean winds drive the surface waters 

 from one climatic zone to another, so that 

 heat transferred from the atmosphere to the 

 water is sometimes transported by currents 

 from one area to another. In turn the air 

 above the water may be warmed and this heat 

 transported again by the winds. This chart 

 shows conditions during the month of 

 February. (See page 261 for prevailing winds 

 chart.) 



203 



