362 PEECT SLADEN TRUST EXPEDITION. 



South of the Equator during the time of the winter monsoon — tlie southern summer — 

 low-pressure areas prevail in Central Australia and in Central and Southern Africa. The 

 belt of low pi-essure which passes through the equatorial regions right round the globe, 

 and marks the well-known belt of calms to which on either hand the trade-winds blow, is 

 drawn southward in the Indian Ocean at this period of the year under the influence of 

 these low-pressure areas, and the line of lowest pressure takes a slanting course south 

 of the Equator from near the north of Madagascar to Sumatra and thence towards 

 Australia. Another result of these low-pressure areas is that the south-east trade-wind is 

 deflected towards the land, being drawn round to blow as a north-west wind on 

 the north-west coast of Australia, and as a light south-east vrind on the east coast of 

 Africa. 



During the period of the summer monsoon — the southern winter — atmospheric 

 pressure in Australia increases everywhere on advancing from the coast into inland 

 regions, and consequently winds blow from the interior towards the coast. The region 

 of highest pressure extends across the Indian Ocean about latitude 30° S., and from 

 that latitude pressure falls continuously northward to the low-pressure area of Central 

 Asia. Southerly winds sweep across the ocean home into Asia ; the equatorial belt of 

 calms is completely obliterated, and the south-east trade-winds are turned to feed the 

 south-west monsoon. 



To the south of latitude 45° S. the normal atmospheric pressure is low at all seasons. 

 Over this zone the prevailing winds are westerly and north-westerly and are subject 

 to little, if any, variation with the season. 



Currents. 



The prevailing winds of the ocean determine to a very great extent the surface 

 circulation. In the northern part of the Indian Ocean the seasonal cbange of the 

 monsoons causes a corresponding variation in the direction of the surface currents of 

 that region. The winter monsoon blowing from the north-east produces a general 

 south-westerly drift in the open sea and a resulting southerly flow along the south-east 

 coast of the Indian peninsula and the east coast of Africa. The summer monsoon 

 blowing from the south-west completely reverses this movement; the drift in the open 

 sea is now towards the north-east, and on meeting the land it produces a southerly flow 

 along the Malabar coast of India, the coast of Burma, and the Malay Peninsula. 



South of the Equator, between latitude 10° S. and the Tropic of Capricorn, the 

 south-east trade-wind causes a steady westward movement of the water— the South 

 Equatorial Current — towards the east coast of Madagascar and the coast of Africa. The 

 onward flow of the water is there checked, and it turns southward in several branches. 

 One of these flowing along the Mozambique Channel is known at first as the Mozambique 

 Current and off the south of Africa as the Agulhas Current. The Agulhas Bank turns 

 the bulk of the stream from its south-westward direction back to the east, and it is 

 carried across the ocean in the di'ift set up by the " brave west winds " to the south of 

 latitude 45' S. A narrow stream of the Agulhas Current rounds the Cape and joins 

 the Benguela Current in the Atlantic. The strength of the Mozambique and Agulhas 



