OCEANOGRAPHY. 17 



not only raise the sea into waves, but drive the 

 superficial layers of the water over the subjacent 

 layers in one direction. In studying a map of 

 the great ocean-currents, we notice a well-marked 

 one lying to the North of the Equator in both 

 the Atlantic and Pacific. This flows from East 

 to West and follows very closely the lines of the 

 prevailing winds in that region. Similarly in 

 the temperate regions of the Southern Hemi- 

 sphere there is an ocean-current, flowing however 

 in this case in the opposite direction from West 

 to East and so corresponding with the trade- 

 winds of that part of the world. The well-known 

 Gulf-stream of the North Atlantic, although 

 modified in some respects by other more compli- 

 cated causes, also follows for part of its course 

 the general direction of the prevailing winds. 



The currents just described are surface currents 

 only, and do not affect to any great extent the 

 mass of the subjacent waters in the ocean-basins. 

 It is difficult to estimate the depth to which their 

 influence reaches, but it is not probable that it 

 extends more than 200 fathoms below the surface. 

 In addition to these, there is also a series of slow 

 currents in the deep waters flowing in definite 

 directions. In the Tropical regions the waters 

 are constantly being heated by the sun, and 

 passed away by the trade-winds to the North 

 and South and ultimately towards the poles. 

 In their long and complicated journey they are 

 gradually cooled down until, in the regions of 

 the ice-bergs, they reach a temperature just 

 above the freezing point of sea-water. Here the 

 water, being colder and therefore heavier than 

 B 



