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° 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.” 
