40 Temperature of the Ocean. 



severe conditions of a polar climate, the limits of oceanic tem- 

 perature in these latitudes are so narrow as to render additional 

 caution necessary. There is, however, considerable agreement 

 between the observations made by the several explorers 

 who have penetrated into these inhospitable regions, who all 

 assert the discovery of warmer water below the cold surface- 

 stratum, so that the fact seems hardly doubtful ; and the recent 

 experience of the officers of H.M.S. "Challenger" in the 

 vicinity of the Antarctic Circle points in the same direction 

 (Fig. 8, Curve B, and Plates 12 and 13). On theoretical 

 grounds it may be said that the existence of open water in the 

 polar regions, in contact with an atmosphere the temperature of 

 which is generally below freezing-point, shows that warm 

 currents from lower latitudes must find their way into these 

 regions. The constant melting of the ice floating in these 

 warm currents must tend to produce layers or pools of water 

 of a temperature near freezing-point and of a lower specific 

 gravity, which, for a time, must remain at or near the surface, 

 and form strata of a temperature lower than that of the strata 

 beneath. 



It will appear from the previous remarks that the distribution 

 of temperature in the ocean depends, as a general rule, upon a 

 constant supply of heat at the surface and a constant supply of 

 cold at the bottom ; and the temperature-curve will represent 

 a series of gradually decreasing temperatures from the surface 

 towards the bottom, as in Fig. 1. In those regions where the 

 supply of heat is reduced to a minimum, as we find is the case 

 in the higher latitudes, the stratum of cold water will be reached 

 within a short distance from the surface (Fig. 2, Curve C, and 

 Fig. 7), and in some parts of the ocean it may be said to occupy 

 the whole depth of the sea (Fig. 8, Curve A). On the other 

 hand, where, as in the cases mentioned above, the supply of cold 

 is reduced or entirely cut off by submarine barriers, the tern- 



