74 SEA FISHERIES 



cold (42 -8, 44'6, and 46-4), and the waters of the Polar 

 current vary from 46*4 to 31 '8. On the Iceland banks 

 the temperature of the surface water in May is 46*4, 

 and on the edge of the plateau 44-6 and 41. In 

 September, between the surface and 38 fathoms, the 

 thermometer marks 53*6. 



We have still to consider the Atlanto-Saharan banks. 

 Their temperature is anything but constant ; it may 

 be considered as a kind of compromise between the 

 temperatures of the three parallel currents, cold, luke- 

 warm, and warm, which skirt the coast. The cold 

 current, between the i5th and 2ist degrees of North 

 latitude, presents the following characteristics : tem- 

 perature in December, 60-9 ; March, 6274 ; May, 

 64*47 > August, 66*32 ; October, 69*85. This current 

 is the nearest of the three to the mainland. The tepid 

 current, which flows alongside, has a winter temperature 

 of 71*6 ; in summer it rises to 73*47 and 77*03. The 

 warm current, which affects only the open sea, main- 

 tains a constant temperature of 80*6 to 82*4 all the 

 year round. It is easy to understand that the tem- 

 perature at any given point or season may vary very 

 widely. It depends upon the west winds, which drive 

 the warm water into the proper belt of the cold 

 current, or upon the east winds, which produce the 

 opposite effect. Let us always remember, too, that in 

 respect of their latitude the waters of the Atlanto- 

 Saharan banks are decidedly cold. 



II 



Between density, temperature, and depth there exists 

 a relation of capital importance, which may be denoted 

 by the symbol nS<, which represents the weight of a 

 litre of sea-water, S 4 , taken at a depth of n metres, with 



