172 GNATS OR MOSQUITOES — CHAPTER VIII 



spots like Fashoda may become valuable possessions, 

 instead of merely affording worthless incitement to 

 international strife and jealousy, and early graves for the 

 Englishmen or Frenchmen who secure the pestiferous bone 

 of contention. 



Surface drainage on the other hand is more usually a 

 matter of detailed small works, the collective effect of 

 which, however, as in the case of our own Fen countiy, 

 may have a most beneficial effect on the public health. 

 Apart from mere gutters, the most effective means of drying 

 the surface of the soil is the well-known agricultural system 

 of " subsoil drainage." Somewhat similar to this, but less 

 effective as a measure of malarial prophylaxis, is the system 

 of drainage employed on Assamese tea gardens, in which 

 the cultivation is divided into plots by straight drainage 

 cuts some five or six feet deep, and these in their turn are 

 divided and subdivided by progressively shallower cuttings. 

 As a measure for drying the soil to suit the needs of the 

 tea bush they are doubtless all that is required, but owing 

 to the impossibility of accurately grading simple cuttings 

 in the soil, they are a fruitful source of puddles. Still they 

 on the whole appear to be beneficial, as they have un- 

 doubtedly diminished the malariousness of certain estates. 



Influence of the Air. — On this point little need be said 

 as its effects are always rather those of climate than com- 

 position. Its chemical composition, the relative proportion 

 present of carbonic acid or ozone, &c., have nothing to do 

 with the case ; but still the word malaria can hardly be 

 said to be a complete misnomer, for it is the air that carries 

 the Mosquito which is the actual vehicle of the malarial 

 germ. The limits of distance to which they can be con- 

 veyed are, however, as we have seen very limited, and for 

 practical purposes the quality of the air of a place may 

 be left out of consideration. 



Ivfluence of Soil. — The comparative freedom of certain 

 sites from malarial disease has been well known from 

 the earliest times, though owing to ignorance of the 

 actual underlying causes, all attempts at generalisation 

 were rendered futile by the constant cropping up of 

 perplexing exceptions. 



