OF THE KERGUELEN REGION OF THE GREAT SOUTHERN OCEAN. 351 



southern icebergs, aud it will be observed that the salinity of the surfiicc water 

 decreases as a colder latitude is approached, and that, generally speaking, there is a rise 

 of salinity with increase of depth. It is therefore probable that the bottom water in the 

 deeper regions of the Southern Ocean is a mixture of water cooled to a low temperature 

 in these regions with water of a higher temperature drawn in, or driven in, from a 

 warmer latitude. 



The effect produced on a sea when its surface is frozen over is an important 

 consideration in discussing these relations of temperature and density. Sea- water ice is 

 composed of a mixture of ice, salt crystals, and mechanically inclosed brine, so that 

 ocean water is divided by freezing into two saliuiferous parts — one liquid, one solid — 

 which»are of different chemical compositions, a striking feature of the freezing process 

 being that the mixture of ice and salt crystals is richer in sulphates than the brine, 

 while the brine is richer in chlorides. In the act of freezing, then, the sea-water 

 separates into ice which contains less salt and into brine which contains more salt than 

 the parent sea-water, and it may be assumed that both the ice and brine have the 

 same temperature (29° F.). The brine being denser than the surrounding water, sinks 

 into it, and by mixing with it renders it more salt and slightly different in composition 

 from the original water, and, at the same time, lowers its temperature. 



Circulation of Ocean Water. — In the portion of the Southern Ocean traversed by 

 the Challenger, the soundings show that there is only a very slight and gradual shoaling 

 from the Indian Ocean towards the Antarctic Circle. Hence there is no impediment to 

 the free circulation of the water between cold and warm latitudes. The effect of the 

 winter cold in the far south is in one respect the same as that of heat in tropical regions 

 — it removes water from the sea and thus produces concentration. In the tropics the 

 water is removed as vapour ; in the polar regions it is removed as ice, leaving a salter 

 water, at the freezing temperature of the sea-water, which sinks and cools the deeper 

 water by convection. In summer, when the ice breaks up, some of it melts and forms a 

 layer of less saltness but low temperature at the surface. This layer, along with the 

 melting pack-ice and icebergs floating in it, is generally driven in part far to the 

 northward of the place where it was formed. Its place must be supplied from below by 

 water coming from more northerly latitudes, unless the supply of land ice from the 

 Antarctic Continent were sufficient to supply the deficiency. 



In the Atlantic, Indian, and Pacific Oceans the return currents of dense warm tropical 

 water which run southward along the eastern shores of South America, Africa, and 

 Australia, penetrate southwards into the region of the Great Southern Ocean, and the 

 effect of these currents can evidently be traced in the distribution of the southern ice at 

 • its northern limits. The water of these currents has such a high salinity that it can bear 

 much dilution and still sink through the water of cold latitudes at the same temperature. 

 It is very probable, therefore, that the cold w^ater of the bottom of the ocean within the 

 tropics mostly comes from the Southern Hemisphere, and leaves the surface between the 

 parallels of 42° and 56° of south latitude. From this ;jone the water is drawn northward 



VOL. XXXVIII. PART II. (NO. 10). 3 B 



