eS EITECTS OF FORESTS ON HITMIDITT. 



windward side, a comparatively precipitous fall to leeward. The 

 observatory erected by Sir Joha Hersuliel with a view to secaring 

 calm for the immease tube of his telescope was erected ia relation to 

 prevalent winds to windward, not to leeward, of Table Mountain. 

 And a calm prevails at Herschel and Bishop's Court while the wind 

 is blowing strongly on the Flats, and pouring over the front of 

 Table Mountain like an avalanche, 



' To some to whom the fact may be new, it may prove corroborative 

 of what has been stated if I call attention to another fact, which has 

 been observed by others. In walking along the edge of a sea-cliff 

 facing the wind it has been found, when the breeze was slight or 

 moderate, that it was felt in full force up to the edge of the preci- 

 pice ; but when the breeze was strong, there was a breadth of perfect 

 calm along the ed;^e of the cliff, while a little way inward the blast 

 was felt in all its fury, — the width of the calm being proportionate to 

 the force of the wind, the upward concave curve of the leeward current 

 passing into a convex curve carrying it over the head of the observer. 



It is in connection with my advocacy of the conservation and 

 extension of forests at the Cape of Good Hope as a remedial measure 

 against the aridity — and progressing aridity— of the soil and climate, 

 that I have brought this subject under consideration. I am quite 

 aware that an argument in favour of the cutting down of forests, 

 woods, and trees, as well as an argument for the conservation and 

 extension of them by plantation, may be founded on the fact to 

 which I am calling attention. It is the case that there are places 

 in which the one operation may be as advisable as the other may be 

 in others : there are countries, in which woods and moisture are both 

 in excess, the clearing away of forests to a certain extent is advocated 

 on the very grouad that their effects are such as I am endeavouring 

 to show that they are. But this only goes to strengthen my 

 argument and establish the fact which I am endeavouring to ex- 

 pound, irrespective of the practical application of it which may bo 

 made by myself or others : that fact is, that belts of wood, and 

 fences, and forests, in their several degrees, acting as windbrakes 

 retard both to leeward and to windward the desiccation of ground by 

 the evaporation promoted by drying winds. 



Sect. III. — On, the General Phenomena of Evaporation from Forests, 



It appears to be unquestionable that by the shade which they 

 occasion, and by the shelter against drymg winds which they aSbrd, 



