LAND TEADE- WINDS. 253 



I 



CHAPTEK VI. 



THE TRADE- WINDS OF THE CONTINENTS.— THE MONSOONS. — ETESIAN WINDS. 



The trade-winds, as we said, have not the same regularity on the 

 continents as over the seas. On the surface of the ocean the masses 

 of moving air are not arrested by any obstacle ; they are propagated 

 freely towards the equatorial zone, and can scarcely be turned from 

 their route by the attraction of any marine'^centre of heat, as the 

 temperature of the water only increases or diminishes very slowly, 

 and the oscillations of the thermometer from day to night do not at- 

 tain 36 degrees Fahr. In the midst of the large islands and the con- 

 tinents it is no longer so. Their mountain-chains oppose the 

 course of the winds, and cause them to change their direction ; 

 forests, prairies, sheets of inland waters, plateaux with long slopes, 

 hilly countries, large plains, and the innumerable variations of topo- 

 graphical relief, are variously heated by the sun, and by this very 

 circumstance turn aside or repel the wind which blows from the 

 neighbouring seas. In the higher regions the current can, it is true, 

 continue its normal movement above the plateaux and the moun- 

 tains ; but below the uneven surface of the country is traversed by 

 irregular winds. Here the band of equatorial calms is completely 

 obliterated, there it is enlarged in an abnormal manner ; the winds 

 are deflected variously on one side or the other, and are directed 

 towards that country whose air is most expanded by the rays of the 

 sun. Nevertheless, it must be said that only a very insufficient num- 

 ber of meteorological observations have as yet been made in the 

 greater part of tropical countries. 



Still we cannot doubt but that the trade-winds blow over vast con- 

 tinental tracts, as well as over the surface of the seas. In fact, the 

 want of rain and the almost complete absence of vegetation in all that 

 part of Africa known by the name of the desert of Sahara, prove in an 

 indubitable manner the existence of a regular wind from the north- 

 east. After having passed the high plateau of Asia, and having dis- 

 charged itself of the greater part of its watery vapour, this atmospheric 

 current traverses obliquely the whole of Africa from the banks of the 



