THE PHYSICAL CONDITIONS OF THE ABYSS 31 



determined, the deduction from the known facts of 

 physical geography that they do actually exist is 

 perfectly sound and beyond dispute. A few con- 

 siderations will, I think, make this clear. 



If the ocean were a simple basin somewhat deeper 

 at the equator than at the poles, the cold water at 

 the poles would gradually sink down the slopes of 

 the basin towards the latitude of the equator, and 

 the bottom temperature of the water would be 

 constant all the world over. 



A few hills here and there would not affect the 

 general statement that for a constant depth the tem- 

 perature of the lowest stratum of water would be 

 constant. 



But in some places ridges occur stretching across 

 the oceans from continent to continent, and these 

 ridges shut off the cold water at the bottom of the 

 sea on the polar side from reaching the bottom of 

 the sea on the equator side. 



If A (fig. 1) represents a ridge stretching from 

 continent to continent across an ocean, and the arrow 

 represents the direction of the current, then the water 

 that flows across the ridge from the polar side to the 

 equator side will be drawn from the layers of water 

 lying above the level of the ridge, and consequently 



