30 THE OCEAN. 



and rises; while that from the polar regions moves 

 in to supply its place: thus a nothern and southern 

 current are produced towards the equinoctial. But 

 the earth is revolving from west to east, and the 

 equatorial parts are, as we have before seen, those in 

 which the velocity is greatest: the free air cannot 

 at once acquire this velocity, and is left 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 cur- 

 rents, makes the direction of the northern one 

 north-east, and of the southern south-east. The 

 point directly beneath the sun, also, is continually 

 travelling westward, which increases the effect. The 

 heat radiated from the surface of 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 become 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 lati- 

 tude, 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 Oc- 

 tober 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 



