62 On the Laius which regulate the 



butary, the Atbera, in its long course of about 2000 miles 

 over a dry and rainless desert. The other known rivers of 

 Africa, the Niger, the Senegal, the Gariep, and the Zambeze, 

 all rise within the limits of the tropical and sub-tropical zones. 

 We may suppose that the great watershed of Africa exists 

 near the centre, and extends from west to east. The most re- 

 cent discoveries on this continent indicate high mountains near 

 the equator ; and north and south of these are lakes and rivers, 

 in all likelihood derived from those snow-peaked summits. 



The Hydrology of Australia presents anomalies apparently 

 more connected with the formation of the land- surface than 

 the condition of the atmosphere. The absence of mountain 

 ranges, especially in the northern half, prevents the forma- 

 tion of rivers, by being unfavourable to the condensation of 

 atmospheric moisture, while the evaporation from the low, 

 level, and arid surface of the interior carries off all the rain 

 that falls, so that the only river system of the country is in 

 the mountain range of the south-east shores. 



It would appear that, in the lower or tropical and sub-tro- 

 pical latitudes, the presence of snow-capped mountains is es- 

 sential to the full and permanent supply of rivers ; and it is 

 thus that the Andes and Rocky Mountains in America, the 

 Tibetan and Himalaya ranges in Asia, the Alps and their 

 connected ranges in Europe, the Pyrenees and other peaked 

 summits of the Iberian Peninsula, and the Urals, intermediate 

 between Europe and Asia, — which latter contribute largely to 

 the Volga and other streams of the great central basin — 

 constitute the main centres of supply for the principal rivers 

 of the world. 



We accordingly lind that the most celebrated mountain 

 peaks, as well as the greatest amount of elevated land on the 

 earth's surface, lie within 40° degrees of the equator, both 

 on the north and south. The highest summits within this 

 range of latitude are from 25,000 to 28,000 feet. A few peaks 

 beyond the latitude of 40° attain elevations of 10,000 to 17,000 

 feet ; but the general tendency of the mountain ranges and 

 table-lands is to decline towards the poles; and the vast 

 Russian plains in the northern hemisphere, and those of the 

 Pampas and of Patagonia in the southern, are evidences of 



