140 A TEXTBOOK OF OCEANOGRAPHY 



but little over wide limits. Consequently it is possible to 

 determine the existence of such a current e.g., the Gulf 

 Stream or Labrador Current by thermometrical and aero- 

 metrical observations alone. 



THEORIES OF OCEAN CURRENTS. 



Early theories of the cause of ocean currents were for the 

 most part fantastical. One idea was that ocean currents were 

 attributable to cosmic influences. Anciently it was thought 

 that all ocean currents had a westerly trend, and this was 

 attributed to the rotation of the earth and the gravitational 

 attraction of the moon. 



Other theories attribute ocean currents to differences of 

 water temperature, to an interaction of the vis inertia of the 

 water and the rotation of the earth, or finally to the prevailing 

 winds. 



An attractive theory is that which attributes ocean currents 

 to differences in specific gravity of sea-water caused by 

 differences in temperature and salinity. Amongst modern 

 investigators this theory is supported by Nansen. That it is 

 not a new idea is, however, shown by the fact that it was held 

 in the time of Columbus by Leonardo da Vinci. 



Seafaring men, as a rule, believe that currents are mainly 

 caused by wind, and though some influence is doubtless exer- 

 cised by the wind, it will not by itself account for all ocean 

 currents. Some physicists doubt the power of the wind to set 

 in motion large volumes of sea-water. 



Alexander von Humboldt (1816) gave a clear account of 

 the possible factors causing ocean currents. These were, 

 briefly, the temperature and salinity of the water, the 

 periodical melting of Polar ice, variable evaporation at 

 different places on the ocean surface, and, finally, differences 

 in atmospheric pressure. 



Sometimes these causes work in conjunction, sometimes 

 in opposition. Modern ideas take into consideration many 



