494 Mr Crossland, The Coral Reefs of Zanzibar. 



the equatorial current of the Indian Ocean meeting the African 

 continent 1 . The tidal currents cause variations in Zanzibar 

 channel, as they run in from the north and south, meeting about 

 half-way along the west coast. It is continuous to the north in 

 the south-west monsoon, but in the north-east monsoon at springs 

 the tidal currents overpower it in all the small channels and 

 harbours. 



The south-west monsoon commences in March, and its first 

 two to three months are occupied by the greater rains. The 

 lesser rains fall in October and November, the north-east monsoon 

 beginning at their close. About a week of variable winds occurs 

 at the change of monsoons. The south-west monsoon therefore 

 lasts nine months, the north-east three. The annual rainfall 

 averages sixty inches. 



The wind is somewhat variable near the land, becoming easterly 

 every afternoon, and as it reaches its greatest strength usually 

 about this time most wind comes from the south-east. 



A continuous heavy swell, such as is found on the west coast of 

 Africa and on the Pacific Islands, does not occur. The surf is due 

 mainly to local winds, and is nearly always present. Indeed I only 

 remember two or three calm afternoons during my stay of a year, 

 though the mornings were often calm. Only one hurricane is 

 recorded, i.e. in April 1872, when a destructive cyclone came 

 from the north-east, but the south end of the island escaped. 

 Ordinary storms are so rare that a well-found open sailing-boat 

 will hardly ever be weather-bound in these seas. 



The 80° F. isothermal lines for the surface-water of the ocean 

 enclose 40 G of latitude in the Indian Ocean. Near the African 

 coast this belt narrows to about 30° of latitude, and Zanzibar 

 lies about its centre 2 . As the winds and currents are from the 

 east warm surface-water bathes these shores continuously. 



Structure of the Island. 



Zanzibar stands between latitudes 5° 42' S. and 6° 28' S., its 

 length being 46 miles, its breadth 20 miles and its distance from 

 the African mainland about the same. 



The hundred fathom line lies one mile from the reef edge in 

 the northern half of the east coast, and two and a half miles in 

 the southern half. The chart records no soundings at a distance 

 from land except in the channel between Zanzibar and Pemba 

 island, where a depth of 450 fathoms is marked. The chart 

 gives the bottom as rock or sand, usually the latter. 



1 Hugh R. Mill, "The Realm of Nature." 



2 Isothermal chart. Hugh R. Mill, "Realm of Nature." 



