TXFLUKNC'K OF FoUKSTS ON iNaTUHAL WaTKK-JSl Pl'LV. I'M 



A vorv interestiiii;' fact in eojinectioii with forests is that it is an 

 extremely (hingcrous thing foi- an aeronaut to pass at a low elevation 

 over the crest of a mountain that is heavily -wooded on the other sicle, 

 as the balloon would at once descend. The same thing occurs when 

 ballooning over rivers and lakes, and results from a tendency to a 

 descending movement of the air and the increased density of the gas 

 caused by the cooler atmosphere. The air is colder over forests, and 

 this prism of cold, humid air is said by Mouillefert to attain a height 

 of from 1000 to 1500 metres. In connection with the relation of 

 forests to rainfall, it is interesting to note the reputed effect on 

 hail-storms. 



It is stated by Mons. P. Mouillefert that numerous (observations 

 made in different places prove that forests have a great intiuence on 

 hail-storms, and that wooded districts suffer less damage from hail 

 than others.* Another recent Fi'ench author, while admitting that 

 the recorded observations afford a strong argument in support of the 

 useful action of forests on hail-storms, says that absolutely convincing 

 facts are still too few in number for a definite conclusion to be 

 formed.! 



The design for all irrigation works from rivers is vitally affected 

 by the regime of the flow. The headworks that would he suitable for 

 perennial rivers of slight fluctuation might be quite inadequate when 

 the flow varied from a small stream to violent freshets, rising 30 ft. 

 and over in a few hours. Such cases are quite common in South 

 Africa, and in some districts — as, for instance, in the lower reaches 

 of the Sunday River in Cape Colony — practically all the available 

 water during the growing season is supplied by the intermittent floods 

 caused by thunderstorms in the mountains long distances away. In 

 such cases the intakes have 130 be specially protected, and, as the floods 

 rapidly subside, it is necessary to have canals of lai"ge carrying capacity 

 so as to distribute the water while it lasts. The irregularity and vio- 

 lence of floods vastly increase the cost of storage reservoirs, not only 

 for waste weir construction, but also for holding capacity to equalise 

 the uncertainty of supply. The larger the proportion of the catch- 

 ment areas which is shaded by forest vegetation, the more favoui'able 

 and sustained will be the water-supply. Here, then, is a mission for 

 forestry and engineering to undertake. Mr. Buckley mentions the 

 value of well-wooded catchment areas, and attributes the fairly con- 

 stant discharge of the Suri-uli Naddi and its tributaries to the fact 

 that they draw their supply from mountains which are overgrown 

 with impenetrable forests.; 



Then for the irrigation engineer, whose interests extend beyond 

 mere questions of construction, and who takes an active interest in 

 promoting the duty of water after it has been delivered on the fields, 

 the question of the growing ti'ees for wind-breaks is of no small im- 

 portance. It is claimed that on level land they are effective to a 



Trnite de Sj/li-'icaltwre. \ Economic Forest iihx, by (x. Hutiel. 



Irriffotioii in Indui and Enmit, by R. B. Buckley, M.I.C.E., p. 71. 



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