182 Mr. J. Croll on the Physical Cause of Ocean- currents. 



chart indicate correctly the direction of the flow, we must be- 

 come convinced that the Gulf-stream water cannot possibly 

 avoid passing through the Gibraltar Strait. Of course the excess 

 of evaporation over that of precipitation within the Mediterra- 

 nean area would of itself produce a considerable current through 

 the Strait ; but this of itself would not fill that inland sea to 

 overflowing. 



The Atlantic may, in fact, be regarded as an immense whirl- 

 pool with the Saragossa Sea as its vortex; and although it is 

 true, as will be seen from an inspection of the Chart, that the 

 wind blows round the Atlantic along the very path taken by the 

 water, impelling the water forward along every inch of its course, 

 yet nevertheless it must hold equally true that the water has a 

 tendency to flow off in a straight line at a tangent to the circular 

 course in which it is moving. But the water is so hemmed in 

 on all sides that it cannot leave this circular path except only at 

 two points ; and at these two points it actually does flow out- 

 wards. On the east and west sides the land prevents any such 

 outflow. Similarly, in the south the escape of the water is frus- 

 trated by the pressure of the opposing currents flowing from 

 that quarter ; while in the north it is prevented by the pressure 

 exerted by polar currents from Davis Strait and the Arctic Ocean. 

 But in the Strait of Gibraltar and in the north-eastern portion 

 of the Atlantic between Iceland and the north-eastern shores of 

 Europe there is no resistance offered ; and at these two points an 

 outflow does actually take place. In both cases, however, espe- 

 cially the latter, the outflow is greatly aided by the impulse 

 of the prevailing winds. 



The consideration that ocean-currents are simply parts of a 

 system of circulation produced by the system of prevailing winds, 

 and not by the impulse of the trade- winds alone, helps to remove 

 the difficulty which some have in accounting for the existence of 

 under currents without referring them to difference of specific 

 gravity. Take the case of the Gulf-stream, which passes under 

 the polar stream on the west of Spitzbergen, this latter stream 

 passing in turn under the Gulf-stream a little beyond Bear 

 Island. The polar streams have their origin in the region of 

 prevailing northerly winds, which no doubt extend to the Pole. 

 The current flowing past the western shores of Spitzbergen, 

 throughout its entire course up to near the point where it dis- 

 appears under the warm waters of the Gulf- stream, lies in the 

 region of these same northerly winds. Now why should this 

 current cease to be a surface- current as soon as it passes out of 

 the region of northerly into that of south-westerly winds ? The 

 explanation seems to be this : when the stream enters the region 

 of prevailing south-westerly winds, its progress southwards along 



