•258 Transactions of the South African Philosophical Society. 



There are no records of observations from Kuruman or from points 

 further to the north or west, but from an examination of the curves 

 of mean annual rainfall, which trend somewhat north and south, 

 there appears to be a considerable falling off in the amount of rain 

 as one proceeds towards the Southern Kalahari. 



This rainfall occurs almost entirely during the four summer 

 months, and for the rest of the year the amounts registered are 

 comparatively insignificant. 



There can be no doubt that in former times, within the last 

 century, the rainfall over this portion of South Africa was much 

 greater than that of the present day ; this is a matter of the highest 

 importance. 



Though the testimony of the earlier travellers is neither unani- 

 mous nor conclusive, the existence of river gravels at levels far above 

 the present river-beds, and the deep cuttings made by the rivers 

 themselves through ridges of hard rock, show that at the present 

 day the rate of excavation and erosion is very much smaller than 

 was the case in times past. 



Whether the present diminution is merely a temporary one, to be 

 followed by a gradual return to former conditions, is not by any 

 means evident ; but it will be best in our considerations to ignore 

 such a possibility. 



Of all this quantity of water that falls upon the surface of the 

 ground, none of it, if we except the Dry Harts Eiver, makes 

 its way to the sea ; that is, the run-off over this area is nil. 

 Since practically there is no loss in this way, the whole of the 

 rainfall will have to be accounted for by the processes of storage, 

 evaporation, and transpiration ; and while the addition of water 

 to the subsoil can take place during a very small fraction of 

 the year only, its removal goes on day after day throughout that 

 period. 



(2) Penetrative power of water. — It is a commonly noticed fact 

 that after a thunderstorm of the average intensity the soil is still 

 quite dry at a depth of a few inches below the surface, the precise 

 distance varying for different soils. 



The downward passage of the rain is greatly influenced by the 

 porosity of the soil, i.e., the ratio of the space between the sand- 

 grains to the volume occupied by the sand itself ; the larger these 

 grains are the more rapidly does the water sink down. During, and 

 immediately after, rain capillary action aids gravity, but as soon as 

 the surface soil becomes drier than that below the action is reversed 

 and the flow due to capillarity is upwards. 



Hence the immense importance of periods of prolonged humidity, 



