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 froni 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 
