INTEODUCTION. 19 



behind ; the effect being that an apparent motion in 

 the contrary direction is given to it, which, combining 

 with the one already possessed by the polar currents, 

 makes the direction of the northern one north-east, 

 and of tlie southern south-east. The point directly 

 beneath the sun, also, is continually travelling west- 

 ward, which increases the effect. The heat radiated 

 from the surface cf large masses of land being superior 

 to that from the sea, while the former is subject to 

 much variation from differences of elevation, and other 

 circumstances, the trade-winds are disturbed, and be- 

 come very irregular in the vicinity of land ; but in 

 open sea they blow with much precision. 



A singular deviation from the uniformity of the 

 trade-winds occurs in the Indian Ocean, which it 

 seems difficult to explain. From 30= south latitude, 

 to within about 10° of the equator, the trade is pretty 

 constant from the south-east ; but to the north of 

 the latter parallel, the wind blows six months from 

 the north-east, namely, from October to April, while, 

 during the remainder of the year, from April to 

 October, it blows with equal pertinacity in a direction 

 diametrically opposite. These are called respectively 

 the north-east and south-west monsoons; but the 

 former is the regvrlar trade — the latter alone is the 

 anomaly, and needs explanation. The cause usually 

 assigned is, the rarefaction of the air on the continent 

 of Asia during the summer months, when the sun is 

 north of the equator ; the air from the Indian Ocean 

 flowing in to supply its place. Tliis would suffi- 

 ciently explain why the wind should be southerly, but 

 leaves its westerly inclination entirely unaccounted 



c 2 



