Distribution of Tropical Diseases in Africa. 485 
the diffusion of the poison or in preventing it exercising its 
potent effects. Wind may carry the malarial poison from a 
marsh to a distance of some two or three miles. Malaria 
may rise to a height of 600 or 700 feet in a calm atmosphere ; 
wind will prevent this vertical diffusion. 
Water can convey the malarial poison, but it is unknown 
at present how far it can carry it. 
The poison is ponderable, and affected by barometrical 
pressure, and it is possible also that food may be con- 
taminated by it. 
The influence of jungle and forest on malaria must also 
be noticed, because so mueh of Central Africa is covered by 
one or other. In a jungle, malaria is intensely virulent, and, 
owing to want of ventilation by the penetration of winds, 
it is there in a very concentrated form. In forests the 
production of malaria is to some extent lessened by the 
shade, and by the trees diminishing the amount of rainfall 
reaching the soil. There is no doubt that forests often act 
as a screen or filter, and therefore protect the district from 
malaria when they lie between it and a marsh. 
With regard to the prevention of malaria, much may be 
done by careful drainage, not only of the surface, but of the 
sub-soil water. Great care should be exercised in the choice 
of a residence, ravines being avoided, also the neighbourhood 
of swamps. Settlements, and even individual houses, should 
be on the most elevated situations, and it should be remem- 
bered that malaria is less rife in the centre of towns, « 
especially if the streets are narrow and crooked. The 
proposal to build houses in the form of a hollow square 
is to be commended, and in all cases they should be 
constructed with a blank wall to the prevailing wind, 
especially if that wind blows over a marsh. The thick 
jungle in the neighbourhood of a settlement should be 
destroyed, but care should be taken not to remove either 
thickets or trees between a settlement and a marsh. The 
ground under and around a habitation should be rendered 
impervious to water and air, and the sleeping rooms should 
be in the second storey. In camping out even, considerable 
protection may be obtained by sleeping in a mosquito curtain 
