444 Proceedings of the Royal Physical Society. 
the rainy season sand-flies and mosquitoes are especially 
troublesome. 
The harvest season or autumn is the most unhealthy part 
of the year in West Africa. The rains moderate, the swamps 
begin to dry, while the decomposing vegetable matter favours 
the spread of malaria. The heat, though not actually so 
great as in summer, is far more oppressive from the moisture 
of the atmosphere. The south-west monsoon is the prevailing 
wind, and at the end of the season the north-east monsoon, 
while beyond the region of the “trades” there are the usual 
land and sea breezes. At the termination of the harvest the 
electrical condition of the atmosphere is much disturbed, this 
being followed by the cessation of rain for some months. 
The harmattan or cold season is so called from a wind of 
very peculiar character which occurs at this time. It is cold 
and extremely dry owing to its course from the east over the 
Sahara. On its approach, vegetation of every kind is shrivelled 
up, and the lips and eyes of those exposed to it suffer from its 
parching effect. It is accompanied by a thick fog and mist, 
composed of particles of fine sand of the desert. It only 
blows for a few days in the season, the prevailing wind being 
from the south-east. This is the dryest season of the year, 
and is generally healthy; vegetable matter being shrivelled 
instead of decomposing as during the autumn, does not favour 
the production of malaria. 
The months of February, March, April, October, and 
November may be regarded as the hot season of West Africa. 
During this period the increased heat gives rise to certain 
physiological effects on the human body. The temperature 
rises to 100° F. or even 102° F., the pulse is accelerated, the 
respirations diminish in depth and frequency, the urine 
becomes concentrated, and activity of the skin is enormously 
increased. 
Although in the summer season the heat is most intense, 
it is not so unhealthy as the autumn, when the heat 
is combined with moisture. (The month of October is 
specially unhealthy.) The heat at first causes an exaltation 
of general sensibility, but afterwards this gives way to marked 
depression. | 
