38 THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN [PART I 



irregular band between the equatorial stream region and latitude 

 47° where the mean annual temperature varies from29''C. tol5'5°C., 

 and (3) a region extending from the northern limit of (2) to within 

 the Arctic circle where the mean annual temperature varies from 

 15*5° C. to 4*5° C, this latter area including the British Islands. It 

 will also be apparent that the isotherms do not run parallel with 

 the lines of latitude, even roughly in such a way as we see is the 

 case in south temperate seas, but north of latitude 40° are bent to 

 the north-east, and south of this parallel are less strongly bent to 

 the south-east. This disposition is due entirely to the influence of 

 the Gulf Stream and the north-easterly drift of Atlantic water, and 

 to the much less abundant southerly flowing polar stream which is 

 evident on the shores of North East America. 



In the North Atlantic, in the axis of the Gulf Stream drift W. 

 from Ireland, the annual range of temperature is 5° C. This is 

 naturally much less than is observed in the shallow seas such as 

 the North, or Irish Sea, where the cooling and heating of the 

 land, and the entrance of water from rivers, have a great effect on 

 the temperature of the sea. In the North Sea we have extreme 

 yearly variations of 2° C. to 17° C, and in the Irish Sea of 8° C. to 

 18° C. Where there are shallow shores, and strong tides, the sea 

 off the land may be strongly heated or cooled; for as the foreshore 

 is laid bare by the tide it is heated or cooled, and water flowing 

 over it is distributed by the tidal currents. 



Quite another series of variations are to be observed if we 

 study the vertical distribution of the temperature of the sea. 

 This may be done by making " hydrographical soundings " in the 

 manner described in the last chapter. We have seen that an 

 elevation of the ocean bed extends across from Scotland to the 

 Island of Greenland, and this ridge forms a " barrier " which is of 

 the greatest significance in the vertical distribution of the tempera- 

 ture of the sea. On one side of this barrier the sea slopes down 

 into the depths of the Atlantic Ocean, and on the other it deepens 

 to form the basin of the Norwegian Sea. Within comparatively 

 short distances of the Wyville-Thomson Ridge we find that the 

 Atlantic has a depth of 3000 metres, while in the Norwegian Sea 

 there is an extreme depth of about 3700 metres. Water currents 

 tend to flow along deep depressions and we find that the Wyville- 



