544 TROPICAL HYGIENE, SANITATION, ETC. 



years. Whooping-cough of uncertain causation shows no marked 

 periodicity, but tends to become epidemic every two years on an 

 average. 



WATER. 



WATER— AMOUNT. 



One should estimate for about 15 gallons per head per diem for 

 personal and domestic use, made up as follows : — 



Domestic washing... ... ... ... 4 gallons 



Ablutions ... ... ... ... ... 3 „ 



General baths ... ... 4 ,, 



W.c 4 M 



One should allow for a little waste in excess of this. 



In towns ID gallons per head should be allowed daily for municipal 

 purposes, and if trade processes are at all numerous another 10 gallons 

 should be added. 



Hence, roughly, one may say provide : — 



12 gallons per head per diem where there are no w.c.'s. 

 16 „ „ ,, are w.c.'s. 



36 „ ,, in large towns generally 



WATER— SOURCES. 



All natural waters, springs, rivers, lakes, &c., are derived ultimately 

 from rainfall, which is water taken up from humid surfaces of the 

 earth, distilled under the influence of the sun's rays and distributed 

 to the earth afresh. 



Part of the rainfall is quickly evaporated. Some of it flowing 

 above an impervious layer finds its way into shallow springs, streams 

 and lakes. The remainder sinks below the surface below an imper- 

 vious layer, when it may form deep springs. 



Bodies of water may lie in large " pockets " either just above or 

 below an impervious stratum ; in the former case surface wells can be 

 dug, and in the latter deep wells will be required in order to obtain 

 it. A deep well, however, may not be so deep as a shallow well, the 

 difference being according to the nearness to the surface of the imper- 

 vious layer. 



Ninety per cent, of rain water sinks below the soil surface in sandy 

 and gravel districts, while a clayey soil permits of an exceedingly small 

 filtration. 



In places where rainfall is scanty, as at Aden, or only salt water 

 is available, as on the ocean and on salt lakes, drinking water is 

 obtained by the distillation of the salt water. 



This process is carried out for the European residents and troops 

 at Aden, for the miners on some Australian mines and on board war 

 and other vessels. 



