SECT. XIII. STRATUM OF CONSTANT TEMPERATURE. 101 



Cape of Good Hope. But the ice which encircles the south pole 

 extends to lower latitudes by 10 than that which surrounds the 

 north. In consequence of the polar current Sir Edward Parry 

 was obliged to give up his attempt to reach the north pole in the 

 year 1827, because the fields of ice were drifting to the south 

 faster than his party could travel over them to the north. 



Kotzebue and Sir James Eoss found a stratum of constant tem- 

 perature in the ocean at a depth depending upon the latitude : at 

 the equator it is at the depth of 7200 feet, from whence it gradually 

 rises till it comes to the surface in both hemispheres about the 

 latitude of 56 26', where the water has the same temperature at 

 all depths ; it then descends to 4500 feet below the surface about 

 the 70th parallel both in the Arctic and Antarctic Seas. The 

 temperature of that aqueous zone is about 39'5 of Fahrenheit.* 

 It divides the surface of the ocean into five great zones of tem- 

 perature, namely, a medial region, in which the highest mean 

 temperature is 82 Fahr., two temperate zones each of 39'5 

 Fahr., and two polar basins at the freezing point of salt water. 



* See the chapter on the Tides and Currents in the ' Physical Geography,' 

 by the author, 4th edition. 



