i9o6] ' OF DEEP-SEA CIRCULATION. 41 



diffusion and mechanical mixture, water of superior gravity is 

 derived and that this controls the abysmal circulation. 



Dr. Otto Pettersson, in an elaborate article, supports by experi- 

 ment and observation the theory of Bjerknes, that tlie melting of the 

 polar ice also promotes circulation, both superficial and deep-seated,, 

 but I can only make reference to this here.^ 



A survey of the existing temperatures and salinities of the ocean 

 also 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 two hundred to three hundred 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 ta 

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

 that the concentrated warm saline waters form inverted cone-like 

 masses that reach down some four thousand 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 strik- 

 ing 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 den- 

 sity due to evaporation is more potent than freshening by precipita- 

 tion, in harmony w^th theory as already set forth, and that the den- 

 sity due to salinity is not greatly over-matched by the low tempera- 



^ " On the Influence of Ice-melting on Oceanic Circulation," Geog. Jour., 

 XXIV., 1904, pp. 285-333. 



-"The Norwegian North Polar Expedition, 1893-1896, Scientific Results," 

 Fridjof Nansen, Vol. II, Oceanography of the North Polar Basin. 



3 " Challenger Reports," Summary of Results, Pt. II, Appendix, Rept. on. 

 Oceanic Circulation. 



