484 Proceedings of the Royal Physical Society. 
the rains cease and give place to warm dry weather. An 
endemic outbreak of malaria and its epidemic spread are 
both notably diminished at the height of the rains, if they 
are very abundant, but the malarial process is developed 
more abundantly in wet than in dry years. These remarks 
are well illustrated by the behaviour of malaria in different 
districts. In Equatorial Central Africa, where the rainfall is 
fairly equally distributed throughout the year, the amount of 
the disease remains practically the same, but in regions, eg., 
along the White Nile to the north of Lado, where there are 
two wet seasons, a rise and fall in the production of malaria 
is manifest. But it is not alone rainfall which influences the 
production of the disease. Drainage from rivers, lakes, and 
pools, periodical or irregular inundations, and the height of 
the sub-soil water, influence its production. This last point 
is of importance, because it explains the occurrence of malaria 
in localities remote from river basins, in the Sahara, in 
Darfur, ete. 
Although the geological characters of the country would 
appear to exert little or no influence on the production of 
the disease, it is the contrary with the physical characteristics 
of the soil. Clay, loam, clayey marl, and marshy soil are 
most favourable to the production of the disease. A porous 
chalky soil is less favourable, and a sandy soil least so, 
provided that they do not rest either upon clay or firm rock. 
Again, the greatest amount of malaria will be found where 
the organic matter in the soil is greatest. It is also an 
undoubted fact that changes in the soil, produced by cultiva- 
tion or its neglect, influence the production of the disease. 
In well-cultivated countries malaria disappears, and if marshy 
districts are well drained or completely covered with water, 
the disease is also diminished. 
The configuration of the ground also causes a local effect, 
for it is found that the disease is more virulent in the lowest 
altitudes ; even the difference of 50 or 100 feet in altitude 
. Ina plain makes a considerable difference as to the salubrity 
or otherwise of a given spot. 
Winds act only indirectly on malaria, as, for instance, by 
moderating temperature; they may, however, act directly in 
