420. Proceedings of the Royal Physical Society. 
east at lat. 16° N., and having its apex at lat. 28° N., long 0°. 
Apart from this, with the exception of Morocco, Algeria, 
Tripoli, and the Nile delta, an oblong area commencing in 
Abyssinia, and running down to the equator, Cape Colony, 
Matabeleland, Damaraland, and Angola, where the mean 
annual temperature is between 60° and 70° F., the remaining 
part of the continent has a mean annual temperature of from 
75° to 80° F. From this it will be seen that, although 
along the northern, western, and southern coast-line the 
temperature is to some extent lowered by the proximity of 
the ocean, yet on the east the sea exerts no modifying effect, 
but practically increases the temperature 5° to 10° F. 
With regard to the mean annual range of temperature in 
Africa, considerable variations exist. There is a district which 
may be roughly said to embrace nearly the whole of the 
Congo Free State, where the range is less than 5° annually, 
and in the Sahara we arrive at the other extreme, the range 
being there between 40° to 50°. Practically, from lat. 18° 
N. to 20° S., the annual range is from 10° to 20°. With 
regard to relative humidity, it is over 70 per cent. in about 
a third of the continent. The upper boundary of this area 
may be described as a line drawn from Cape Verd to Lado 
on the White Nile on the north, bounded by 52° E. long. in 
the east and lat. 15° S. in the south, and including, also, the 
coast belt from Cape Town to lat. 2° N. on the east coast. 
Apart from this, with the exception of Abyssinia, where the 
humidity is also over 70 per cent., it is considerably less. 
The rainfall in Africa likewise varies greatly, it being 
under 5 inches in the Sahara and in the Kalahari Desert. It 
is over 100 inches at Sierra Leone and between Lagos and 
Gaboon. Over the remaining part of the continent it varies 
from 10 to 100 inches. Speaking generally, the annual rain- 
fall may be estimated at from 50 to 60 inches. The rainfall 
at the equator is pretty evenly distributed throughout the 
whole year, but to the north of the equator; at about 15° lat., 
there are two well-marked rainy seasons. 
With reference to winds, the trade winds are the most 
important. North of the equator the prevalent trade winds 
are from the north-east, south of it from the south-east, with 
