p. 7 



vessels' log books had given the navigator a rough picture of such 

 of the ii-ajor currents as affected hi^i the most, especially in the 

 North Atlantic. And while it was chiefly by suiting the sailing 

 routes to the prevailing winds that the use of Maury's sailing direct- 

 tions expedited voyages, ^ it is certain that advantage taken of the 



1. The average voyage from England to America was shortened by 

 about ten days thereby, from England to Australia and return by 

 about sixty days. 



prevailing current was partly responsible for the fact that British 

 ship masters alone are said to have profited ten million dollars 

 annually by using Maury's and more recent current charts, so long as 

 sailing ships continued to carry the bulk of the world's commerce. 

 And while full-powered steamers now run more independently of the 

 current, co.itinued collection of such data has been considered so 

 important that the Hydrographic and Meteorologic offices of Great 

 Britain, of Germany, of Holland, and of the United States had to- 

 gether gathered more than 27,000,000 notes on the wind, weather, 

 surface temperature, and surface drift of the sea, up to 1904. 



Notv/ithstanding the vast amount of data assembled, even the 

 surface currents can yet be pictured only in a very generalized \;ay. 

 This is due in part to the nature of the information available from 

 log books, in which any leeway that the ship may have made is 

 usually included in the recorded "drift." But a more serious 

 difficulty is that ocean currents do not progress like smooth flowing 

 rivers, but are constantly varying in velocity and direction, eddying, 

 even terporarily reversed by the wind, in a way so complex that it 

 is not yet :oossible to state the details for any part of the sea, 

 at any season of the year. Furthermore, surface data, taken by them^ 

 selves, may give a very erroneous picture of the actual circulation, 

 because a knowledge of the rr.ovements of the underlying water is 

 equally essential. In this last respect, reports from passing ships 

 do not help us at all. 



Since it has not proved feasible to use current meters frequently 

 enough in deep water, or at stations enough, to be of much value 

 there, it is necessary to turn to indirect methods to learn the 

 direction of the horizontal flow in the deeper layers. The sorts of 

 data from which the latter may be deduced are various. The distribu- 

 tion of oxygen gives us some information. So does the distribution 

 of the different kinds of sediments on the bottom; also the geo- 

 graphic distribution of various plants and animals. But far the 

 most reliable indices to the movements of v/ater-masses belov; the 

 surface are their temperatures and salinities. Hence it v;as not until 

 a satisfactory deep sea thermometer was invented, and an accurate 

 method of measuring the salinity (or the specific gravity, for the 

 one can be calculated from the other) , that science was in a posi- 

 tion to gather eL".piric knowledge of the movements that geographers 

 had long postulated for the bottom waters of the ocean. 



We are able, by expansions of cold or of warm water, of high 

 salinities or of low, to recognize arctic or tropical currents; 

 land water fanning out off river mouths; the lines of dispersal for 

 the highly saline waters that result from evaporation at the surface 



