146 A TEXTBOOK OF OCEANOGRAPHY 



6. In some instances a bottom ciirrent bas been observed, 

 notably by Tizard in 1882 on the Wyville-Thomson ridge, 

 between Scotland and the Faroes. This ridge séparâtes a cold 

 and warm area. 



The warm Atlantic surface water flows north, and the cold 

 water found on the northern side of the ridge is this water 

 which bas cooled and sunk and then flowed down to the ridge. 

 The resuit is that there is cold water on both sides of the 

 ridge, but on the north of the ridge the decrease downwards is 

 much greater than on the south. The form of the isothermal 

 lines in the figure is that of a volume of water flowing over a 

 weir (from right to left in the illustration). Not only are there 

 theoretical reasons for this cold current, but as a fact the 

 current on the ridge is sufïiciently strong to keep it clear of 

 sédiment. 



CURRENTS IN N ARROW WaTERS. 



As a rule currents in narrow waters Connecting océans and 

 seas are due to différences in sea-water density. In high 

 latitudes, where the rainfall is high and evaporation relatively 

 low, fresh water plays a part, causing a raising of the water- 

 level. The converse holds good in lower latitudes. In the 

 absence of wind and tide — i.e., on quiet days at neap-tides — ■ 

 two distinct currents are observed in narrow straits, the upper 

 moving to the région of water of higher density ; that is, in 

 the Straits of Gibraltar and Bab el Mandeb from the océan to 

 the sea, and conversely in the Bosporus and in Cabot Strait. 

 The rotation of the earth causes a deflection of the stream to 

 the right in the Northern Hémisphère, so that in the Straits of 

 Gibraltar it is bent to the Moroccan coast. 



In most straits, especially those which open to the océan, 

 there are strong tides, and thèse tend to obscure the currents. 

 Observations for the détermination of a current should extend 

 for a period of thirteen hours, to cover a complète tide. 



