400 



HYDROGRAPHY. 



ocean, or. the distance to the coast of Africa, is 2000 miles; — a place 

 on the coast of America would be two hours in passing through this 

 arc. The difference is therefore three hours, and the retardation of 

 the vapor to the west about 400 miles an hour. 



As the effect of the sun decreases, these rains diminish, until they 

 cease altogether, when the dry season ensues. The atmosphere being 

 no longer surcharged with vapor, and the land more heated, precipita- 

 tion does not take place. The vapor, then, which is held suspended, 

 passes onward to the high mountain ranges of the western coast, where 

 it is condensed, and supplies the extensive rivers of South America. 



That the vapor is derived from the east, is well established, from 

 the fact that condensation is more evident when the current of air 

 flows from that direction ; and it is shown by the prevalence of more 

 copious rains, which take place on the eastern side of mountains and 

 on the west side of seas and lakes. The Caspian Sea is a well-known 

 instance of this. On the east side it is a barren, sandy waste, and 

 rain rarely falls; while on the west it is fertile, with much rain. The 

 rainless districts may also be cited as an evidence that vapor passes 

 over a high plateau, such as the deserts of Gobi, Iran, and Syria, where 

 the temperature is elevated, and the vapor cannot undergo condensa- 

 tion; and that it does not, is proved by the fact that metals of all kinds 

 never oxidate when exposed to the atmosphere ; that vast quantities 

 of vapor pass over them, we know, for it is condensed on the Hima- 

 laya Mountains, and supplies the large rivers of Asia with their waters. 

 The cause of the rainless and barren district, stretching entirely over 

 the African Continent, 3000 miles from east to west, by 1000 miles 

 wide, north to south, may also be readily accounted for. The Hima- 

 laya Mountains have condensed the vapor flowing over them from the 

 east ; the cold and dry air is thence induced onward to the heated 

 areas of the Sahara, having its capacity for moisture greatly increased. 

 On its route it takes up all the moisture which evaporation has left, 

 leaving it perfectly dry and unsusceptible of cultivation. While this 

 continues, it must ever remain a desert. 



But it is not my intention to deny that vapor may permeate towards 

 the east; its tendency, as before remarked, is to distribute itself to- 

 wards the land, and its province is to restore the equilibrium in all 

 directions. This tendency is increased by the wind, particularly 

 when it prevails from the water to the land ; its action, however, is 

 very limited. The great precipitation which is observed on many 



