'28 Transactions of the South African Philosophical Society. 



coast, 'is almost as humid as the coast, whereas O'okiep, fifty miles 

 inland from the west coast, is exceedingly dry, and become so 

 because the high temperature of the land as compared with the sea 

 has lessened the relative humidity. This principle was known at 

 end of the eighteenth century at least. Dalton has the following 

 characteristically sagacious remarks upon the rainfall conditions of 

 the north-west of England : " The reason that a S.W. wind in these 

 parts brings rain seems to be that, coming from the torrid zone, it is 

 charged with vapour, and the heat escaping as it proceeds north- 

 ward, a precipitation of the vapour ensues ; but a N.E. wind, 

 blowing from a cold into a warmer country, has its capacity for 

 vapour increased, and therefore we generally find it promote 

 •evaporation." * 



The interpretation of these facts seems to be that our rain 

 originates on the equator, being carried hither in the upper 

 atmospheric currents flowing from the west and north. The rain 

 begins on the east coast when this moist upper stratum meets the 

 lower bodies of air damp with moisture from the Indian Ocean. 

 Then it gradually works back from the east coast as the eastern air 

 becomes heavier with vapour, f 



Now the upper current, though it may be nearly saturated at its 

 low temperature aloft, does not increase its absolute humidity by 

 commingling with the air of the west coast, and therefore can flow 

 across the slopes bordering the tableland without being induced to 

 condense its moisture. 



The copious rainfall of the district extending from Ceres to Table 

 Mountain is perhaps largely promoted by its proximity to the 

 region where the warm Agulhas and cold Benguela currents meet. 

 Wojerkof mentions this cold current in connection with the aridity 

 of the West Coast: — "An der Kiiste riihrt diess zum Theil von der 

 kalten Meeresstromung her, welche in der Nahe fliesst und von 

 welcher aus kalte S.W. — und Slid Winde nach der Kiiste wehen. 

 Die Passatwinde von der Ostkiiste kommen iiber das hohe Binnen- 

 land her schon in einem sehr trockenen Zustand an" ("Die 

 Atmospharische Circulation, &c," in Erg. No. 38. Geo. Mitt. 1874). 



* Dalton, " Meteorological Observations and Essays," 2nd eel., 1834, p. 132. 



f The month of greatest average rainfall over Natal is December (see Table 25), 

 whereas the quantity of moisture in the air of Durban is greatest in February. At 

 Durban itself, October, with a mean moisture of 5 # 8 grains per cubic foot, has as 

 great a daily rainfall as February, with a mean moisture of 7*5 grains. 



