SECT. XXV. TEMPEKATURE OF THE LAND. 243 



perature at noon ,and midnight is about 1*37, the greatest de- 

 viation never exceeding from 0'36 to 2 0> 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 dryness. From the 

 eastern extremity of the Sahara desert quite across Africa, the soil 

 is almost entirely barren sand ; and the Sahara desert itself 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 ther- 

 mometer to descend 12 or 14, and that in the meadows and 

 heaths in England the absorption of heat by the grass is suffi- 

 cient 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 eva- 

 poration 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 Orinoco, 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. However, the frigorific effects of their evaporation are 

 counteracted 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 

 caloric, and cool the air by evaporation ; but, on account of the 

 chilled and dense particles sinking to the bottom, deep water 

 diminishes the cold of winter, so long as ice is not formed. 



In consequence of the difference in the radiating and absorbing 

 powers of the sea and land, their configuration greatly modifies 



M 2 



