&6 PHYSICAL GEOGRAPHY OF THE SEA. 
retard the flow of rivers, and thus it is to be expected that con- 
tinuous winds, even of moderate strength, must have a tendency 
to impel the waters in the same direction. 
The steady trade-winds of the tropical zone, and the prevail- 
ing westerly winds in higher latitudes, consequently unite their 
influence with that of the above mentioned causes, in driving 
the waters of the tropical seas to the west, and those of the 
temperate zones to the east. 
The tides also, which on the high seas generally move from 
east to west, promote the flow of the ocean in the same 
direction, and thus contribute to the westerly current of the 
tropical seas. 
Nor must we forget that the obstacles which the ocean- 
currents meet on their way; such as intervening lines of coast, 
sand banks, submarine ridges, or mountain chains, have a great 
influence upon their course, and may even give them a dia- 
metrically opposite direction to that which they would otherwise 
have followed. 
Having thus briefly mentioned the origin and causes of the 
currents, which intersect the seas like huge rivers, I shall now 
describe such of them as are most important and interesting in 
a geographical point of view. 
In the northern part of the Atlantic, between Europe, North 
Africa, and the New World, the waters are constantly perform- 
ing a vast circular or rotatory movement. Under the tropics 
they proceed like the trade-winds from east to west, assisting 
the progress of the ships that sail from the Canaries to South 
America, and rendering navigation in a straight line from Car- 
thagena de Indias to Cumana (stream upwards) next to im= 
possible. This westerly current receives a considerable addition 
from the Mozambique stream, which, flowing from north to 
south between Madagascar and the coast of Caffraria, proceeds 
round the southern extremity of Africa, and after rapidly ad- 
vancing to the north, along the western coast of that continent, 
as far as the island of St. Thomas, unites its waters with those 
of the equatorial current, and continues its course right across 
the Atlantic. In this manner the combined tropical streams 
reach the eastern extremity of South America (Cape Roque), 
where they divide into two arms. The one flowing to the south 
follows the south-eastern coast, and vradually takes a south- 
