PHYSICAL CONDITIONS OF THE DEEP SEA. 467 



above it. That is to say, it would be warmer at the equator than 

 in the temperate regions. The temperature at the bottom would 

 be the same as the lowest temperature of the basin, that is, of the 

 earth that supports it. The great oceans, however, can not be re- 

 garded as simple basins of water such as this. The temperature of 

 the surface water varies only approximately with the latitude. It 

 is, generally speaking, hottest at the equator and coldest at the 

 poles, but surface currents in the intermediate regions produce 

 many irregularities in the surface temperature. 



Again, although we have no means of knowing what the tem- 

 perature of the earth is at one thousand fathoms below the surface 

 of the ocean, it is very probable that in the great oceans the tem- 

 perature of the deepest stratum of water is considerably lower than 

 the true earth temperature. This is due to currents of cold water 

 constantly flowing from the poles toward the equator. If these 

 polar currents were at any time to cease, the temperature of the 

 lowest strata of water would rise. Although the polar currents 

 can not be actually demonstrated nor their exact rapidity be accu- 

 rately determined, the deduction from the known facts of physical 

 geography that they do actually exist is perfectly sound and be- 

 yond dispute. A few considerations will, I think, make this clear. 



If the ocean were a simple basin somewhat deeper at the equa- 

 tor than at the poles, the cold water at the poles would gradually 

 sink down the slopes of the basin toward the latitude of the equa- 

 tor, 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 temperature of the 

 lowest stratum of water would be constant. But in some places 

 ridges occur stretching across the ocean from continent to conti- 

 nent, 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 con- 

 tinent 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 lay- 

 ers of water lying above the level of the ridge, and consequently 

 none of the coldest water will ever get across it, and from the level 

 of the ridge to the bottom of the sea on the equatorial side the 

 water will have the same temperature as the water at the level 

 of the ridge on the polar side. It follows from this that in j)laces 

 where there are deep holes in the bed of the ocean surrounded on 

 all sides by considerable elevations, the temperature of the water 

 at the bottom will be the same as the temperature of the water 

 on the summit of the lowest ridges that surrounds them. 



This explains why it is that we find that the bottom tempera- 

 ture for a given depth is frequently less in one place than it is in 



