256 THE ATMOSPHEEE AND METEOEOLOGT. 



South Sea. The circulatory system does not in any place pass be- 

 yond the lower strata of the aerial ocean, and we may easily perceive 

 above the islands of Sunda and Australia, as well as over the sides of 

 the Himalayas, the constant progress of the clouds which are brought 

 by the regular trade- winds. A volcano of Java, observed by Jung- 

 huhn, affords a remarkable example of this. From its summit, about 

 9900 feet high, a column of vapour escapes all the year round, which 

 bends gracefully in space, and directs itself towards the west, or north- 

 west, in a long whitish cloud, and it is in precisely the opposite direc- 

 tion that the monsoon blows during six months of the year, on the 

 slopes as well as at the foot of the mountaios. 



The monsoons of the East Indies are not the only winds which 

 break the uniformity of the trade-winds. In all those parts of the 

 tropical zone where the shores of the continents are disposed parallel 

 to the equator, the winds alternate regularly, in consequence of the 

 greater rarefaction of the air which occurs now on the earth, now on 

 the sea, according to the position of the sun. Thus, during the 

 greater part of the year, the African coasts, which stretch from the 

 Bight of Benin to Cape Palmas, attract the monsoons of the Gulf of 

 Guinea. These masses of air changing their direction, turn back to 

 blow in a north-easterly direction, and rush rapidly towards the great 

 furnace of the Sahara, where the overheated atmosphere is usually 

 more expanded than in any other country of the world. Towards 

 the month of January, when the Sahara itself has become colder than 

 the equatorial seas and the banks of the Congo, the trade-wind of 

 the north-east re-assumes the supremacy, and traverses the whole of 

 Northern Africa obliquely to the south, towards the coasts of Southern 

 Guinea. Yery violent at first, it soon becomes weaker, and hardly 

 lasts but two or three weeks, when it again gives place to the marine 

 monsoon. During its short prevalence the current coming from the 

 deSert does not cease to bring with it a white dust having the appear- 

 ance of a thick fog. It is the sand of the Sahara, which in the 

 regions situated immediately to the north of Guinea is almost white, 

 while further the dust raised from the ground by the Harmattan is 

 nearly red.* 



On the coasts of Chili, on those of California, in the islands of the 

 Pacific, around the Gulf of Mexico and the sea of the Antilles, analo- 

 gous phenomena occur. In summer the valley of the Mississippi and 

 the plateaux of Texas are traversed by real monsoons, which dis- 

 tribute the rain over that part of the continent, and then are in turn 

 * Borghero, Bulletin de la Societe de Geographie, July, 1866. 



