254 THE ATMOSPIIEEE AND METEOEOLOGY. 



Nile to those of the Niger. On this enormous track of nearly 3000 

 miles it only lets rain fall on some mountain-summits, such as the 

 Djebel-Hoggar, and scarcely casts a single cloud on the unchanging 

 azure of the sky. On the western coast of the Sahara, the burning 

 wind called the Hannattan is nothing else than the north-east trade- 

 wind more or less turned from its course because of the neighbourhood 

 of the sea. Towards the 17th degree of north latitude, on the south- 

 ern frontiers of Soudan, clouds are at last formed in space, abundant 

 rains penetrate the soil, and the aridity of the desert gives place to 

 a fine vegetation ; this is, because the domain of the permanent winds 

 terminates there, to be replaced by the zone of equatorial calms with 

 an ascending current loaded with aqueous vapours. In the southern 

 part of Africa, the trade- winds of the south-east make themselves re- 

 gularly felt, and according to the testimony of Livingstone, traverse 

 the entire continent from the mouths of the Zambesi to the coast of 

 Angola. On the other side of the great strait of the Atlantic, the 

 tropical regions of South America are likewise refreshed by the con- 

 stant breath of moist winds from the south-east. Brazil, Paraguay, 

 a great part of the Argentine republic, Bolivia, Peru, Guiana, and 

 Columbia, are comprised in this great meteorological region. The 

 trade-wind, turned back under the equator in an east and west di- 

 rection, ascends with a uniform force the valley of the Amazons, 

 penetrates into the gorges of the Andes, and even crosses by all the 

 defiles the high barrier of mountains ; but sheltered by this enormous 

 rampart the shores of the Pacific are not subjected to the influence of 

 the east wind. The vessels which sail in the open sea have to traverse 

 from 125 to 625 miles, according to the latitudes, before a gust of the 

 trade- wind, descending from the summit of the Andes, comes to 

 swell their sails and drive them to the coast of Australia. 



Even in those parts of the world where the tropical winds cease to 

 be permanent, the oscillations and deflections of the atmospheric cur- 

 rent present in general a periodical character and occur regularly, 

 according to the course of the seasons. Among the regular re- 

 turn winds we may cite principally the '' monsoons " of India and 

 Arabia. The Arabian name for these meteoric phenomena, maussim, 

 or moussim, signifies change, season; it is because, in fact, they 

 divide the year in the most exact manner into two totally distinct 

 periods. During the great heats of summer, the arid plateaux of 

 Central Asia, and even the plains of Hindostan, much more heated 

 than the sea, act like a great respiring pump ; the air which rests 

 above this part of the Asiatic continent expands, and in consequence 



