CONDITIONS INFLUENCING PEEVALENCE 185 



SO are distinctly anti-malarial. In subtropical regions on 

 the contrary, especially far inland, the problem the cultivator 

 has to solve is how to keep the surface soil damp enough 

 to keep his crops alive. 



Subsoil drainage w^ould double the labours of his well 

 bullocks, and to prevent the escape of the share of the scanty 

 showers that fall on his patch of ground he subdivides his 

 holding into small, carefully levelled patches, each but a few 

 yards square, surrounding each patch with a low ridge of 

 earth a few inches high. 



Naturally m such regions cultivation favours malaria. 



Irrigation may be divided into two kinds : {a) Where the 

 water is raised to the surface by water lifts of various kinds ; 

 and (6) where it is brought on to the land by gravitation, or 

 in other words, by canals. 



The first is always a domestic operation ; the second can 

 only be successfully carried out by large and expensive 

 engineering works. Whichever plan be adopted, the 

 method of applying the water to the land is the same. The 

 cultivator having levelled and subdivided his land in the 

 manner described above, the water is made to flow in turn 

 into each little square until the ground within its bound- 

 aries is covered to a depth suitable to the particular crop. 

 In practice (except for rice) the amount given is usually 

 absorbed in a few hours, and the careful levelling of each 

 patch is distinctly unfavourable to the formation of puddles. 



In domestic irrigation the water is raised from wells by 

 bullock or human labour, or it may be scooped up in adroitly 

 swung baskets from rivers or swamps, but whatever its 

 source, the water is too laboriously gained to be wasted, 

 either in leakage or puddles. The quantity raised is usually 

 too small to perceptibly affect the level of the ground water; 

 but so far as it goes the tendency nmst be to lower the 

 spring levels, and on the whole I believe that the infinences 

 of domestic irrigation are rather unfavourable to malaria 

 than otherwise. 



In the case of canal irrigation the results are quite 

 different, as the enormous quantity of water used usually 

 seriously raises the level of the groimd water and may even 



