SKCT. XXVI. TEMPERATURE OF THE LAND. 257 



of all these circumstances, the ocean is not subject to 

 such variations of heat as the land ; and by imparting 

 its temperature to the winds, it diminishes the rigor 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 

 degree of latitude, the surface of the ocean is in gene- 

 ral warmer than the air above it. The mean of the 

 difference of the temperature at noon and midnight is 

 about l-37, the greatest deviation never exceeding from 

 0-36 to 2'16, which is much cooler than the air over 

 the land. 



On land the. temperature depends upon the nature 

 of the soil and its products, its habitual moisture or dry- 

 ness. From the eastern extremity of the Sahara 

 desert quite across Africa, the soil is almost entirely 

 barren sand ; and the Sahara desert itself, without in- 

 cluding Dafour or Dongola, extends over an area of 

 194,000 square leagues, equal to twice the area of the 

 Mediterranean Sea, and raises the temperature of the 

 air by radiation from 90 to 100, which must have a 

 most extensive influence. On the contrary, vegetation 

 cools the air by evaporation and the apparent radiation 

 of cold from the leaves of plants, because they absorb 

 more caloric than they give out. The graminiferous 

 plains of South America cover an extent ten times 

 greater than France, occupying no less than about 

 50,000 square leagues, which is more than the whole 

 chain of the Andes, and all the scattered mountain- 

 groups of Brazil. The'se, together with the plains of 

 North America and the steppes of Europe and Asia, 

 must have an extensive cooling effect on the atmosphere 

 if it be considered that in calm and serene nights they 

 cause the thermometer to descend 12 or 14, and that 

 in the meadows and heaths in England the absorption 

 of heat by the grass is sufficient to cause the tempera- 

 ture to sink to the point of congelation during the night 

 for ten months in the year. Forests cool the air also 

 by shading the ground from the rays of the sun, and by 

 evaporation from the boughs. Hales found that the 

 17 Y 2 



