POSSIBLE REVERSAL OF DEEP-SEA CIRCULATION 371 



makes it clear that the battle between temperature and salinity is a 

 close one, and that no profound change is necessary to turn the balance. 

 The combined results of the many polar expeditions have shown that 

 in the high latitudes of both hemispheres there is a superficial sheet 

 of water 200-300 meters deep that is colder, but lighter, than that 

 below, because it is fresher. It floats upon a warmer, more saline 

 body of water below. This has been specially demonstrated by the 

 investigations of Nansen.^ This layer of coldest water moves to 

 lower latitudes superficially in the main, showing that coldness alone 

 is not determinative. 



In the open Pacific and Indian Oceans hydrostatic equilibrium 

 must be very closely maintained, because of the slight resistance to 

 adjustment. It is shown by the charts of Dr. Alexander Buchan^ 

 that the concentrated warm saline waters form inverted cone-like 

 masses that reach downward 4,000 feet or more. It thus appears 

 that they lie in the same horizons as great masses of colder 

 waters which their salinity must counterbalance. Less striking 

 phenomena of similar import mark the evaporating areas of the 

 north and south Atlantic. The equatorial tracts of freshened waters 

 arising from high precipitation are scarcely traceable to half the 

 depth. This seems to imply that in the low latitudes increased 

 density due to evaporation is more potent than freshening precipi- 

 tation, in harmony with theory, as already set forth, and that the 

 density due to salinity is not greatly overmatched by the low tem- 

 perature density of the Antarctic regions, from which the Pacific and 

 Indian Oceans are not separated by appreciable barriers. 



An interesting illustration of the close balance between salinity- 

 density and temperature-density is presented by the saline waters 

 that issue from the Mediterranean, in which evaporation is in excess 

 of combined precipitation and inflow from adjacent lands. As a 

 result, the concentrated waters that form the deeper body of the 

 Mediterranean creep out through the bottom section of the Straits 

 of Gibraltar, while the upper section is occupied by a compensating 



^ Fridtjof Nansen, The Norwegian North Polar Expedition, 1893-1896, Scien- 

 tific Results; Vol. II, " Oceanography of the North Polar Basin." 



2 Challenger Reports, Summary of Results, Part II, Appendix, " Report on Oceanic 

 Circulation." 



