242 TEMPERATURE OF THE OCEAN. SECT. XXV. 



the vapours floating in the air, and send them down in torrents 

 of rain. They radiate heat into the atmosphere at a lower eleva- 

 tion, and increase the temperature of the valleys hy the reflection 

 of the sun's rays, and by the shelter they afford against prevail- 

 ing winds. But, on the contrary, one of the most general and 

 powerful causes of cold arising from the vicinity of mountains 

 is the freezing currents of wind which rush from their lofty 

 peaks along the rapid declivities, chilling the surrounding 

 valleys : such is the cutting north wind called the bise in 

 Switzerland. 



Next to elevation, the difference in the radiating and absorb- 

 ing powers of the sea and land has the greatest influence in dis- 

 turbing the regular distribution of heat. The extent of the dry 

 land is not above the fourth part of that of the ocean ; so that 

 the general temperature of the atmosphere, regarded as the result 

 of the partial temperatures of the whole surface of the globe, is 

 most powerfully modified by the sea. Besides, the ocean acts 

 more uniformly on the atmosphere than the diversified surface 

 of the solid mass does, both by the equality of its curvature and 

 its homogeneity. In opaque substances the accumulation of 

 heat is confined to the stratum nearest the surface. The seas 

 become less heated at their surface than the land, because the 

 solar rays, before being extinguished, penetrate the transparent 

 liquid to a greater depth and in greater numbers than in the 

 opaque masses. On the other hand, water has a considerable 

 radiating power, which, together with evaporation, would reduce 

 the surface of the ocean to a very low temperature, if the cold 

 particles did not sink to the bottom on account of their superior 

 density. The seas preserve a considerable portion of the heat 

 they receive in summer, and from their saltness do not freeze 

 so soon as fresh water. So that, in consequence of all these cir- 

 cumstances, the ocean is not subject to such variations of heat 

 as the land, and, by imparting its temperature to the winds and 

 by its currents, it diminishes the rigour of climate on the coasts 

 and in the islands, which are never subject to such extremes of 

 heat and cold as are experienced in the interior of continents, 

 though they are liable to fogs and rain from the evaporation of 

 the adjacent seas. On each side of the equator to the 48th de- 

 gree of latitude, the surface of the ocean is in general warmer 

 than the air above it. The mean of the difference of the tern- 



