140 A TEXTBOOK OF OCEANOGRAPHY 



but Utile over wide limits. Consequently it is possible to 

 détermine the existence of such a current — e.g,, the Gulf 

 Stream or Labrador Current — by thermometrical and aero- 

 metrical observations alone. 



Théories of Océan Currents. 



Early théories of the cause of océan currents were for the 

 most part fantastical. One idea was that océan currents were 

 attributable to cosmîc influences. Anciently it was thought 

 that ail océan currents had a westerly trend, and this was 

 attributed to the rotation of the earth and the gravitational 

 attraction of the moon. 



Other théories attribute océan currents to différences of 

 water température, to an interaction of the vis inertiœ of the 

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

 winds. 



An attractive theory is that which attributes océan currents 

 to différences in spécifie gravity of sea-water caused by 

 différences in température and salinity. Amongst modem 

 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 ail océan 

 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 océan currents. Thèse were, 

 briefly, the température and salinity of the water, the 

 periodical melting of Polar ice, variable evaporation at 

 différent places on the océan surface, and, finally, différences 

 in atmospheric pressure. 



Sometimes thèse causes work in conjunction, sometimes 

 in opposition. Modem ideas take into considération many 



