THE OCEAN FEEDING-GROUNDS 83 



does not exceed 1*9 knots. The reader will see later on 

 what conclusions may be drawn from all these facts. 



For the same reason I will devote a few lines to the 

 vertical currents of which M. Nathansohn has written. 

 There are such currents to the east of the Rejkjanaes 

 ridge (in the south-west of Iceland), in the Straits of 

 Messina, and in general on the borders of hot and cold 

 currents. 1 Vertical movements are perceptible chiefly 

 in the open sea. Near the coast they are hampered by 

 the affluxion of the fresh water of rivers and by the 

 mechanical action of the coast, which causes the layers 

 of water inshore to sink to the lowest depths of the 

 currents which impinge upon it. Natterer has proved 

 the reality of ascending currents in certain parts of the 

 Mediterranean by noting the proportions of nitric acid 

 and bromine contained in the different layers of water. 

 Nathansohn, moreover, analysing the water of the 

 western Mediterranean basin, has often found the same 

 salinity and temperature at the top and the bottom. On 

 April 7, 1909, the Prince of Monaco obtained the follow- 

 ing data between Nice and Corsica : 



Depth. Temperature. Salinity. 



o 54-8 21-32 



7,190 feet 55*34 2 i'3* 



The cause of these ascending currents nearly always 

 resides in the different temperatures of different layers. 

 In summer the sunlight warms the surface layers of the 

 sea. When the winter comes the water of these layers 

 becomes colder and so heavier, and sinks until it finds 

 a layer of the same temperature and the same specific 

 weight. Hence the ascending current. M. Nathansohn 



1 Also in the Atlantic, under the equator, between the Admiralty 

 Isles and the Caroline Isles, between Hawaii and Tahiti, and to the 

 north of Ascension. 



