FORESTRY OF WEST AFRICA. 



on the importance of the effect of vegetation on 

 ground, and especially of the Eiicalyptus globulus : — 



" In hot countries vegetation shades the ground, 

 and makes it cooler. The evaporation from the 

 surface is lessened ; but the evaporation from the 

 vegetation is so great as to produce a perceptible 

 lowering effect oh the temperature of a place. 



"Pettenkofer has calculated that an oak tree, 

 which had 711,592 leaves, had during the summer 

 months (May-October) an evaporation equal to 539. i 

 centimetres (212 inches), while the rainfall was only 

 65 centimetres (25.6 inches) ; so that the evaporation 

 was 8i times the rainfall : this shows how much 

 water was abstracted from the soil, and how the air 

 must have been moistened and cooled. Observations 

 in Algeria (Gimbert) have shown that the EiicalypUis 

 globulus absorbs and evaporates eleven times the 

 rainfall ; extremely malarious places being rendered 

 healthy in this way in four or five years. 



" Alluvial soils. — Many alluvial soils, especially, as 

 lately pointed out by Wenzel, those most recently 

 formed, give out Malaria, although they are not 

 marshy. It is presumed that the newest alluvium 

 contains more organic matter and salts than the older 

 formations. Many alluvial soils have a flat surface, a 

 bad outfall, and are in the vicinity of streams which 

 may cause great variations in the level of the 

 ground water. Mud Banks also, on the side of large 

 streams, especially if only occasionally covered with 



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