OCEANOGRAPHY 413 



from precipitation, which is here in excess of the evaporation from 

 the surface of the sea. 



The volume of Gulf Stream water that is seen in the eastern 

 part (east of Station 10) of the southern Fram section, can only- 

 flow north-eastward to a much less extent, as the Porcupine Bank 

 is connected with the bank to the west of Ireland by a submarine 

 ridge (with depths up to about 300 metres), which forms a great 

 obstacle to such a movement. 



The two volumes of Gulf Stream water in the Fram's southern 

 section of 1910 are divided by a volume of water, which lies over 

 the Porcupine Bank, and has a lower salinity and also a somewhat 

 lower average temperature. On the bank to the south of Ireland 

 (Stations 1 and 2) the salinity and average temperature are also 

 comparatively low. The fact that the water on the banks off the 

 coast has lower salinities, and in part lower temperatures, than the 

 water outside in the deep sea, has usually been explained by its 

 being mixed with the coast water, which is diluted with river water 

 from the land. This explanation may be correct in a great 

 measure; but, of course, it will not apply to the water over banks 

 that lie out in the sea, far from any land. It appears, nevertheless, 

 on the Porcupine Bank, for instance, and, as we shall see later, on 

 the Rockall Bank, that the water on these ocean banks is — in any 

 case in early summer — colder and less salt than the surrounding 

 water of the sea. It appears from the Frithjof section across the 

 Rockall Bank, as well as from the two Fram sections, that this 

 must be due to precipitation combined with the vertical currents 

 near the surface, which are produced by the cooling of the surface 

 of the sea in the course of the winter. For, as the surface water 

 cools, it becomes heavier than the water immediately below, and 

 must then sink, while it is replaced by water from below. These 

 vertical currents extend deeper and deeper as the cooling proceeds 

 in the course of the winter, and bring about an almost equal 

 temperature and salinity in the upper waters of the sea during the 

 winter, as far down as this vertical circulation reaches. But as the 



