164 COLONIAL REPORTS MISCELLANEOUS. 



large extent, determines the distribution of the different types. 

 As one proceeds inland and northwards from the coast, the Har- 

 mattan is found to l>e more pronounced and to last for much 

 longer intervals at a stretch till in the interior beyond the 10th 

 parallel of north latitude it is the prevalent breeze at that season 

 of the year. Rainfall has, of course, the most influence on the 

 vegetation, but at the same time the action is much modified l>y 

 desiccating winds of the nature described above. When these are 

 practically a constant feature of the season for several months at 

 a stretch, the country rapidly dries up, though the actual total 

 rainfall may have been abundant enough to support luxuriant 

 vegetation but for the prolonged dry weather brought about by 

 such winds. Gambaga is a case in point ; the mean annual 

 rainfall, as far as available data go, is 48*21 inches, which is only 

 4*56 inches less than that of Kumasi, a station situated within 

 the evergreen forest belt; but the difference between the mean 

 relative humidity of the air in the two places is very great indeed 

 (>0'41 per cent, in the former place, and 86*27 per cent, in the 

 latter. It is this reduced humidity, brought about by prolonged 

 spells of dry weather, that accounts to a large extent for the 

 very pronounced difference in the vegetation of the two localities. 



There is but little doubt that the presence of extensive forests hi 

 the vicinity of Gambaga would considerably temper and diminish 

 the drying action of the Harmattan winds and make the country 

 moister than it is, though it may result in no very marked im- 

 provement in the rainfall, whilst conversely, the persistent 

 destruction of the forests for a good distance round Kumasi would 

 result in the soil becoming drier, in the water supply being re- 

 duced, in the drying effect of the Harmattans being intensified, 

 and in a change of the type of vegetation. 



Forests act as protective covering to the moisture in the soil : 

 remove them and you diminish the quantity of moisture in the 

 soil. This decrease reacts in its turn on the forest vegetation, and 

 brings about a change in its character that makes it more suited 

 to the altered conditions, and so on the climatic factors and the 

 forests act and react on one another, by mutual adjustments, but 

 the continued destruction of the latter inevitably results in a 

 diminished water supply. 



But beyond this, forest vegetation is responsible for a high per- 

 centage of the total amount of water held in suspension by the 

 atmosphere, and forests exercise a marked effect in increasing the 

 relative humidity of the atmosphere in their neighbourhood. A 

 high relative humidity means moister conditions all round, both 

 as to soil, the water supply of the country, and greater resistance 

 to desiccating influences, by its retarding action on evaporation. 



When it is recollected that the amount of water vapour trans- 

 pired from a large mass of forest vegetation is several hundred 

 times greater than thait evaporated from a water surface of the 

 same area, it will be understood that it does not require a very 

 large portion of the earth's surface to be covered with forest 

 vegetation for the amount of moisture transpired by it to exceed 



