294 TEMPERATURE OF THE LAND. [SECT. xxvr. 



On land the temperature depends upon the nature of the 

 soil and its products, its habitual moisture or dryness. From 

 the eastern extremity of the Sahara desert quite across Africa, 

 the soil is almost entirely barren sand; and the Sahara 

 desert itself, without including 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. These, 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 temperature 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 

 leaves of a single plant of helianthus three feet high exposed 

 nearly forty feet of surface ; and, if it be considered that the 

 woody regions of the river Amazons, and the higher part of 

 the Oroonoko, occupy an area of 260,000 square leagues, 

 some idea may be formed of the torrents of vapour which 

 rise from the leaves of the forests all over the globe. How- 

 ever, the frigorific effects of their evaporation are counter- 

 acted in some measure by the perfect calm which reigns in 

 the tropical wildernesses. The innumerable rivers, lakes, 

 pools, and marshes interspersed through the continents absorb 



