24 



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 Once established, forests appear to exert a reflex action on the distribution of rain. By 



cooling moist air and increasing its humidity, they induce a greater local rainfall. Observa- 

 tions on this point are not numerous. At Cinq-Franchees, in France, Mathieu observed that 

 the rainfall measured in 1866-68, at three points not far apart (one inside a forest, another 

 on the edge, and the third outside), varied respectively as the numbers 1,000, 957, and 

 853. Fautrat and Sartiaut give the results of double observations of the rainfall at 

 Halette, from February to July in one year, as 192'5 mm. in the forest, against 177 mm. 

 outside. The difference appears to be greater in warm than in cold countries ; though a 

 great deal also depends on the meteorological character of the rainfall : whether it is in- 

 duced by cyclones or by secondaries. An increase of the rainfall can only be apparent 

 locally, or in a country of which forests cover a large proportion. In most cases, it would 

 be masked by the greater irregular or cyclical variations from year to year. 



But if forests do not materially augment the rainfall, they unquestionably regulate 

 it, promote the frequency of showers, and control the flow of water, which, on the whole, 

 is a preferable effect. In most countries, certainly in Natal, a simple increase in the inten- 

 sity of rain would be an evil. Forty inches of rain poured down rapidly, will leave a smaller 

 supply in the soil than twenty falling moderately. Torrential rains wasted in flooding 

 the rivers may do some good at times, but frequent soaking rains are better. If forests 

 are a means of reducing the proportion of floodwaters, they give the advantages of an 

 increased rainfall without the attendant evils. 



EROSION OF THE SOIL. In mountain regions, the destruction of forests causes the 

 formation of torrents and ravines. Torrential action is essentially identical in all its 

 varied forms, and consists of the bringing down of soil and stones by the rain waters. 

 Ravines or dongas feed torrents, which in turn give rise to torrential rivers. From the 

 appearance of a river we may judge, of the degree of torrentiality to which it i& 

 subject. When the banks are clothed with vegetation to the water's edge, or 

 green islands in the centre are covered with old trees, great stability and steadiness 

 of flow may be inferred ; on the contrary, signs of erosion and shoals of gravel in 

 the stream indicate a slight degree of torrentiality ; at an aggravated stage, the water 

 may be seen straggling through beds of pebbles ; dividing into branches with unstable 

 courses. A typical torrent can be made out to consist of a fan-shaped hollow basin in 

 which the waters are collected, joined by a narrow gorge to a spreading bed of deposit, 

 the average slope decreasing throughout. The erosion of the ground takes place over 

 the whole basin, but ceases at the gorge, where the material is simply borne along by 

 the water ; finally, the washings become deposited from the foot of the gorge down- 

 wards. 



The erosion of the ground in the torrent basin is caused by sudden heavy rains such 

 as those of thunderstorms. The rain loosens the ground by impact*, the particles of soil 



*Hail has been observed to be especially destructive in this respect. 



