35 



liberated again by combustion, (c) The maintenance of a mantle of vapour which acts, 

 as Tyndall first pointed out, as a screen, (d) The cooling due to evaporation, great in 

 summer but small in winter. 



The increase and regulation of rainfall is due to the reduction of temperature and 

 increase of humidity over a large area. According to Abercromby, clouds will form over 

 rivers and forests when failing to form over the open plains, and at a more intense stage 

 there may be " cloud everywhere and rain only over the rivers and forests." 



Thunderstorms, which are of such frequent occurrence in Natal, appear to be caused 

 by great uprushes of vapour resulting from large differences of temperature in adjoining 

 masses of air, and there are indications that forests, by regulating temperature and 

 humidity, serve to moderate their violence. Abercromby (Weather p. 289) gives a chart 

 of the distribution of hailstorms in the department of the Loiret, in France, during the 

 thirty years, 1836-65, from which it is seen that the villages outside the forest area 

 suffer severely, " while those inside the forest enjoy almost complete immunity from 

 destructive hailstorms." Abercromby says also that " the forest seems to act, in fact, as 

 a breakwater against which the violence of the storms expends itself for a time until it 



can gather fresh force The wind circulation of thunderstorms is always 



confined to the low strata of the atmostphere only When the lower clouds 



meet such an obstacle as a forest, or even a mountain, eddies are formed. The masses of 

 cloud come back on themselves, and seem to be repelled and dispersed by the forest. 

 When the clouds have succeeded in passing the obstacle, their force is exhausted, and 

 they only precipitate rare or inoffensive hail, and do not regain their intensity for some 

 time afterwards." 



Having now discussed the various physical effects of forests, we may see that 

 vegetation in general of which forests are the most intensified form constitutes the 

 natural protection of the earth from^meteorological influences, and it is evident that 

 climatic effects of some moment must result from extensive denudation ; the temperature 

 will increase, and the extremes of cold and heat become more divergent; the aridity 

 becomes greater and the rainfall, and especially the water supply, more irregular ; floods 

 occur, and torrential action tends to regain full scope. In this manner, portions of 

 Western Asia, North Africa and South Europe, which were once populous and fertile, 

 have become waste and arid. More recently, and nearer Natal, the destruction of forests 

 had such marked effects on the climate and salubrity of Mauritius that it was judged 

 necessary to reforest extensively in order to restore the island to its former condition* ; 

 while at Zanzibar, as i was told by Sir John Kirk, extensive denudation has been 



*A Report by Dr. H. Rogers " On the Effects of the cutting down of the Forests on the Climate and" 

 Health of Mauritius," issued in 1871, and a paper read by Dr. R. Meldrum before the Scottish Meteorological 

 Society in July, 1866. supply interesting details. Meldrum estimates that 70,000 acres, or one-sixth of the 

 area of the island, were denuded of timber during the 10 years 1852-62. 



