HYGIENE 533 



RAINFALL. 



Rain is always cooling, and may be abundani in tropical countries. 



Tropical rains are usually very heavy, so that they predispose to 

 floods and the formation of swamps. In India the prevailing winds 

 regulate the rainy seasons. Generally there are two rainy seasons 

 each year between 5° and 10° from the equator, one when the sun 

 is on the zenith and going away from the equator, and the other when 

 it is returning. Otherwise there is but one rainy season during the 

 year. The violence of tropical rains is caused bv the additional 

 amount of water that warm air can take up, thus at : — 



20*^ F. ( 7"5'^ C.) air takes up \';^ grains of water per cubic foot 

 60° F. ds-S" C.) „ S77 



85^' F. (29f/' C.) „ 1278 





When saturated air at 85° F. (29-6 C.) is cooled to 60° F. (15-8° C.) 

 every cubic foot will yield seven grains of water (Hill). 



Owing to the excessive heat and moisture in the tropics, putre- 

 faction and fermentation are very rapid and often dangerous to man. 



SOIL. 



Soil affects the temperature by its power of absorption and 

 radiation and its vegetation. Sandy soils are hottest and emit a glare 

 which is very trying to the eyes. Clayey soils are coolest because of 

 the water they hold. They cause a high atmospheric humidity, retard 

 evaporation and radiation. The climate in consequence is made more 

 ecjuable, but more oppressive and predisposing to rheumatic troubles 

 and chest diseases. 



Soil temperature varies but slightly at a depth of 8 feet below 

 the surface, and is not affected at all at a depth of 40 feet (Galton). 

 Trees and shrubs intercept the sun's rays and check evaporation from 

 the soil and thus render the ground cooler. 



Evaporation, however, is great and takes much moisture from the 

 soil. 



In Algeria the Eucalyptus globulus absorbed and evaporated 

 twelve times the rainfall. Grass renders soil cooler, drier, and of more 

 equable temperature. 



When soil is permanently damp it is injurious to health. In order 

 to keep the subsoil water at a low level subsoil drains must be put in, 

 periodically cleaned and relaid. 



A permanent dampness mav be due to : — 



(i) Clayey soils that do not drain themselves readily, thus forming 

 swampy and mosquito breeding grounds. 



(2) By an impervious layer approaching the surface at a given place 

 and bringing the subsoil water-level higher than it otherwise would 

 be. This level should always be 5 feet below the surface. 



