by an undercurrent flowing down from high latitudes. Throughout 

 most of its journey the undercurrent would flow in an almost 

 horizontal direction, but under the Equator it must well up to the 

 surface. He was aware that the actual directions of such movements 

 might be greatly influenced by the action of the Earth's rotation and 

 by the winds. 



New information about the temperature and salt content of the 

 deep layers led Professor William Carpenter, one of the leading 

 biologists of the 1870s, to say that the ocean circulation was due 

 mainly to heat, cold, and evaporation. He could not see how the 

 wind could cause a deep current. The uncompromising nature of 

 this man brought him into collision with James Croll, an eminent 

 physical geographer who tried to show him that forces due to 

 small differences in density between the polar and equatorial regions 

 could not produce currents as fast as those that were known at the 

 surface. Both men, as it turned out, raised important issues, but 

 neither knew enough about fluid mechanics to get a complete 

 enough grasp of the subject to convince the other. Only today are 

 we beginning to get somewhere with these complex questions. 

 Although there is still plenty of room to argue about the relative 

 importance of wind and density differences, we are beginning to 

 put forward fairly realistic theories that take account of both. 



With this brief historical summary, which shows our constant 

 indebtedness to the earlier scientists and seamen, let us now look 

 in more detail at some of the work being done today and what 

 new information it is revealing about the oceans. 



Between 1889 and 1893, Captain A. Hautreaux 

 plotted the courses of tree trunks and 

 masses of driftwood as they were carried by 

 currents across the Atlantic. The map on 

 this page shows the drift courses of fifty- 

 three objects (compare with map on page 204). 



189 



