LOCALISED CURRENTS. in 



conclusion that instead of there being a permanent deep layer 

 of water at 4° C, the average temperature of the deep sea in 

 temperate and tropical regions is about 0° C, the freezing point 

 of fresh water. 



In the atmospheric ocean, aeronauts not seldom meet with 

 warm air currents flowing above others of a colder temperature ; 

 while, according to a general law, the warmth of the air con- 

 stantly diminishes as its elevation above the surface of the sea 

 increases. 



Similar exceptions to the general rule are met with in the 

 ocean. In moderate depths sometimes the whole mass of water 

 from the surface to the bottom is abnormally warm, owing to 

 the movement in a certain direction of a great body of warm 

 water, as in the "warm area" to the north-west of the Hebrides, 

 where, at a depth of 500 fathoms, the minimum temperature was 

 found to be 6° C. On the other hand, the whole body of 

 water is sometimes abnormally cold, as in the " cold area," be- 

 tween Scotland and Faeroe, where, at a depth of 500 fathoms, 

 the bottom temperature is found to average — 1° C* The only 

 feasible explanation of these enormous differences of tempera- 

 ture, amounting to nearly 13° F. in two areas freely communi- 

 cating with one another, and in close proximity, is that in the area 

 to the north-west of the Hebrides a body of water warmed even 

 above the normal temperature of the latitude flows northwards 

 from some southern source, and occupies the whole depth of that 

 comparatively shallow portion of the Atlantic, while an arctic 

 stream of frigid water creeps from the north-eastward into the 

 trough between Faeroe and the Shetland Islands, and fills its 

 deeper part in consequence of its higher specific gravity. There 

 can be no doubt that similar phenomena occur in various parts 

 of the ocean, and that the deep seas are frequently intersected 

 by streams differing in temperature from the surrounding 

 waters. 



In some places, owing to the conformation of the neighbour- 

 ing land or of the sea-bottom, superficial warm and cold cur- 

 rents are circumscribed and localised, thereby occasioning the 

 singular phenomenon of a patch or stripe of warm and a patch 

 of cold sea meeting in an invisible but well-defined line. 



* " The Depths of the Sea," by Professor Wyville Thomson, p. 307. 



