197 



It may be seen that there are considerable variations of the rainfall in other parts of 

 South Africa, but none nearly so great as that observed at Ottawa, and I cannot but 

 think that the serious diminution in late years of the rainfall of Victoria County is due 

 primarily to the clearing, since 1860, of some 35,000 acres of forest, for coffee and sugar 

 planting. 



There are not many reliable observations of the influence of forests on rainfall. In 

 1867, Dr. E. Ebermayer introduced the use of double stations, one inside a fully stocked 

 forest, the other at some distance outside, both being placed in similar conditions of 

 altitude, aspect, soil, &c. During the last few years the use of these double stations has 

 been extended in Germany and France. The recently available Prussian observations, ** 

 which I did not get in time for insertion in my Forest Report, show the following increase 

 of rainfall in the forest stations over those of the open country, in per cent, of the rainfall 

 of the latter. 



Between sea-level and 328 feet altitude ... ... ... 1'25 per cent. 



Between 328 and 556 feet altitude ... ... ... ... 14'2 



Between 1,969 and 2,297 feet altitude 19 



Between 2,297 and 2,625 feet altitude 43 



These figures have been arrived at from observations taken at sixteen pairs of stations 

 during periods of from four to eleven years. They appear to show that the forests have 

 little influence on the rainfall near sea level, and that their influence becomes considerable 

 with increased altitude in mountainous countries. 



At Nancy, in France, the results of seven years observations made at a pair of 

 stations 1,247 feet in altitude, show a mean increase of 16 per cent, in the rainfall 

 over the forests. 



The action of forests has also been investigated by Swedish observers. I have not 

 the figures before me, but they show that in Scandinavia the influence of forests on the 

 rainfall is much less than in Germany. 



It is probable that the power of the forests to increase rainfall is much greater in 

 warm than in cold countries. The increase of rainfall over forests is due chiefly to the 

 lowering of the temperature in their vicinity. Beech woods in Bavaria reduce the tem- 

 perature by 8*4 in July, while increasing it by 2'1 in January. The effect being most 

 marked in summer, it is probably considerably greater in tropical or semi-tropical climates. 

 A second cause consists of the mechanical action of masses of trees, which break 

 the air currents and induce precipitation. Both causes would act more powerfully, with 

 increased elevation, and this may explain why the excess of rainfall over forests is found 

 to increase with altitude. Conversely, the rains induced by low lying atmospheric 

 disturbances, such as thunderstorms and " secondaries," would be affected more powerfully 

 by the forests than the rains due to " cyclones,"and Abercromby observes this to be the 

 case. The Natal rains being caused by thunderstorms and " secondaries " chiefly, it may 

 be expected that the forests here exercise a powerful influence on the rainfall. 



A third result of denudation, consisting in the increase of radiation from a bared 

 surface, also helps to reduce the condensation of vapour. On a coast region such as the 

 littoral zone of Victoria County, the increased temperature and radiation due to 

 denudation, would probably serve to buoy up over the heated belt, a considerable pro- 

 portion of the clouds from the sea that condense only 15 or 20 miles inland. 



I am inclined to the belief that these three causes are quite sufficient to account for 

 the serious diminution of the rainfall of Victoria County that has followed the destruction 

 of its forests. 



(Signed) H. G. FOURCADE. 

 Durban, February llth, 1890. 



** See Dr. Schlich's Manual of Forestry, vol. I. p. 41. 



