196 A NATURALIST IN MID-AFRICA. 



were first discovered during the Challenger Ex- 

 pedition by the detection of strands of dry air 

 descending. I have myself noticed, just at the 

 time when my porters were trying to prevent my 

 tent from being hurried down the valley, clouds in 

 a high stratum of the atmosphere drifting gently 

 towards the top of the mountain. 



Both this evening wind and the ascent of the 

 cloud are, I think, pretty easily explained. The 

 lower slopes of the mountain are rapidly heated 



by the sun, more rapidly 

 perhaps than the plain, and 

 the hot air rises, carrying 

 with it, and perhaps par- 

 tially dissolving, the cloud 

 At about 6 p.m. the whole 

 side of the mountain is 

 clear of cold air, and the 

 stratum in contact with 

 the snow rushes down to the heated lower ground. 

 I noticed also — and the fact seems to bear out this 

 theory — that when rain is falling or has fallen 

 during the afternoon on the lower slopes, this 

 wind does not blow down the valley with the 

 same force. 



I do not know enough of meteorology to be able 

 to explain several other points which I noticed. 

 For instance, rain is most unusual in the lower 

 levels till the afternoon, and the greatest quantity 

 seems always to fall during the night. Again, 



Fig. 31. — Sea Breeze. 



