PEESIDKNTIAL ADDEJ-:SS SECTIOX C. 59 



trees continue to live yeax* after year, tluit i- evideiue that they 

 are receiving what moisture they require, and that the channel 

 of escape for the moisture into the atmosphere is a more useful 

 one than when the escape was by means of a river into the sea. 



How Much Water is vSaved. 



A very pertinent question is how much water is it possible 

 to save by this means. The savine- in anj- case is the propor- 

 tion of the rainfall which does not run ofi'. The larg^er the 

 area under these vii^orous trees, the gieater is the saving in 

 moisture and the more is the river flow likely to be reduced. 



Then there comes the inquiry : If that be so, how do grass 

 and forest herbage regulate the off-flow required for domestic 

 and economic purposes y It is easj' to see that water retained 

 and prevented from running off during rain obtains thereby 

 an opportunity of sinking into the soil and subsoil. The 

 humus of the forest and the humus of decayed grass foliage 

 alike act as a sponge and keep the water till it soaks in. That 

 water may take da'\'s, weeks, months or years to be all used 

 by the vegetation, but until it is so used a slow and steady 

 infiltration to some stream or underground reservoir is going 

 on — luuch more permanent than the flood-and-drought supply 

 of burned veld, but varying in duration in accordance with the 

 demands of tlie vegetation and the nature of canopy and of 

 humus under which it abides. 



Naturally, a tree of rapid growtli, uiu\ inoducing little 

 humus or canopy when young, may dry the surface soil 

 considerably, but that drying, when it affects the tree, brings 

 down foliage, makes humus, and produces growth in accord- 

 ance with the supply available. 



How Rainfall is Produced. 



But now let us trace that moisture which through 

 tianspiration and evaporation again becomes part of the 

 atmosphere. 



Temperature, atmospheric pressure and altitude govern 

 what humidity the atmosjdiere can carry at any particular 

 place; as these change, so also changes the point of saturation, 

 and as soon as that is reached or passed, de])Osition as rain, 

 mist, dew or snow takes place. Consequently an atmosphere 

 which is capable of absorbing all the moisture that is 

 transpired and evaporated by vegetation many be incapable 

 of carrying that moisture if ii happens to meet a colder current, 

 or rises into a higher altitude where the atmosphere is more 

 rarefied. This accounts for the vegetation being different on 

 rising ground from what it is on the flats below, the precipita- 

 tion on hillsides, and especially on the south-east slopes of the 

 escarpment (i.e., facing the sea breezes) being often sufficient 

 to maintain natural forest which cannot exist elsewhere. 



If all the moisture preciDitated on these slopes ran off at 

 once to the sea, the country behind would get very little, but, 

 as it happens, a very large proportion is transpired and during 

 dry weather absorbed into the atmosphere and lifted till it 

 again arrives at saturation and falls again as rain or mist. 



