MUEEAT — DEPTH AND MAEINE DEPOSITS. 363 



Currents varies according as the north-east or south-west monsoon is blowing, the 

 movement produced by the former reinforcing them, and that produced by the latter 

 weakening them by deflecting northwards a portion of the water and drifting it towards 

 the Indian Peninsula. 



On arriving at the shores of Australia the drift set up by the west winds is divided 

 by the south-west projection of the coast-line into two streams, one of which continues 

 its eastward course along the southern coast, whilst the other, turning northwards, forms 

 the West Australian Current, of which the greater part is drawn into the Southern 

 Equatorial Current on reaching the latitude whence the latter takes its origin. 



In the region of the belt of calms south of the parallel of 5° S. there is a well- 

 marked Counter Equatorial Current. This current flows across the ocean from Zanzibar 

 to Sumatra all the year round, but with greater strength during the period of the 

 south-west monsoon. 



Off the coast of Western Australia the south-east trade-wind blows the surface water 

 westward and causes an upwelling of cold water ; a similar' upwelling takes place off the 

 Somali coast of Africa at the time of the south-west monsoon. 



Tempeeatuke of the Water. 



Surface. — In the Indian Ocean to the north of lat. 13° S. the area of maximum 

 surface temperature (exceeding 80° E.) overspreads the whole of the Indian Ocean, 

 including the Bay of Bengal and a large portion of the Arabian Sea. Over the 

 north-western portion of the Arabian Sea, however, a comparatively low temperature 

 prevails, brought about by the prevailing north-westerly winds there in the summer and 

 also the winter months, which drive the warm surface water to the south-eastwards, and 

 thus bring, by upwelling, the colder waters of lower depths to the surface. The effect 

 is well seen in the mean monthly temperature of the surface of the sea off the Arabian 

 coast about long. 67° E., which during the spring months is 82°-8 E., but falls in the 

 summer months to 76°"3 F., when the north-west winds are strongest, rising again in 

 the autumn to 79°'4! E. This low summer temperature is the moi-e remarkable since 

 the north-west winds which then prevail come from the deserts of Arabia, and are 

 therefore hot, dry, and rainless till they have traversed the sea a considerable way to the 

 south-east. It is also remarkable that coral-reefs are absent along the African coast 

 where this upwelling usually takes place, while to the north and south of this region 

 coral-reefs flourish *. On the otlier hand, the prevailing winds at this season at the 

 head of the Bay of Bengal are southerly and south-easterly ; they therefore blow home 

 on that coast, bringing from tropical regions the warm waters of the surface over the 

 whole of the northern portion of this sea. 



To the south of lat. 13° S. the temperature falls with characteristic regularity as the 

 latitude increases, except that it is rather lower over the eastern parts off the coast of 

 Australia than over the western parts oft' the coast of South Africa. 



* See Murray, "Structure, Origin, and Distribution of Coral Eeefs aud Islands," Eop. Roy. Inst., March 1(5, 1888 

 p. 4 (sep.). 



SECOND SERIES. — ZOOLOGY, VOL. XIII. 49 



