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" Parr Il. Sect. ii. § 6.] OCEAN-CURRENTS, | 491 
prevalent set of the surface waters towards the north-east. As the 
distribution of life over the globe is everywhere so dependent upon 
temperature, it becomes of the highest interest to know that a truly 
- arctic submarine climate exists everywhere in the deeper parts of the 
sea. With such uniformity of temperature we may anticipate that 
the abysmal fauna will be found to possess a corresponding sameness of 
character, and that arctic types may be met with even on the ocean- 
_ bed at the equator. 
But besides this general drift or set, a leading part in oceanic 
circulation is taken by the more defined streams termed currents. 
The tidal wave only becomes one of translation as it passes into 
shallow water, and is thus of only local consequence. But a vast 
body of water, known as the Equatorial Current, moves in a general 
westerly direction round the globe. Owing to the way in which the 
continents cross its path, this current is subject to considerable 
deflections. Thus that portion which crosses the Atlantic from the 
_ African side strikes against the mass of South America, and divides, 
one portion turning towards the south and skirting the shores of Brazil ; 
the other bending north-westward into the Gulf of Mexico, and 
issuing thence as the well-known Gulf Stream. This equatorial 
water is comparatively warm and light.. At the same time the 
heavier and colder polar water moves towards the equator, some- 
times in surface currents like those which skirt the eastern and 
western shores of Greenland, but more generally as a cold under- 
current which creeps over the floor of the ocean even as far as the 
- equator. 
Much discussion has arisen in recent years as to the cause of 
oceanic circulation, ‘Two rival theories have been given. Ac- 
cording to one of these the circulation entirely arises from that of 
the air. The trade-winds blowing from either side of the equator. 
drive the water before them until the north-east and south-east 
currents unite in equatorial latitudes into one broad westerly- 
flowing current. Owing to the form of the land, portions of this 
‘main current are deflected into temperate latitudes, and, as a 
consequence, portions of the polar water require to move towards the 
equator to restore the equilibrium. According to the other view the 
currents arise from differences of temperature (and aecording to 
‘some of salinity also); the warm and light equatorial water is 
believed to stand at a higher level than the colder and heavier polar 
_water; the former, therefore, flows down as it were polewards, while 
the latter moves as a bottom inflow towards the equator; the cold 
_ bottom water under the tropics is constantly ascending to the sur- 
face, whence, after being heated, it drifts away towards the pole, 
and on being cooled down there, descends and begins another 
journey to the equator. There can be no doubt that the winds are 
_ directly the cause of such currents as the Gulf Stream, and therefore, 
indirectly, of return cold currents from the polar regions. It seems 
hardly less certain that, to some extent at least, differences of 
: 
ae > 
2 
