548 TROPICAL HYGIENE, SANITATION, ETC. 



subjected to much pollution. They should not be encouraged 

 especially amongst the native peoples of a country. They form a 

 good growing ground for all kinds of parasites, worms, cyclops,, 

 cholera and typhoid bacillus, mosquito larvae, &c., &c. 



Subterranean Water. 



When rain water falls upon permeable soil much of it percolates 

 through the first impermeable stratum, the depth of uhicli may vary 

 much. The water flows on the surface of this layer in the direction 

 of its fall and may : — 



(i) Become collected in the indentations of its surface and form 

 shallow wells, 



(2) Form a reservoir at the foot of the fall, 



(3) Find a place where the impermeable layer comes to the surface 



when the water passes beneath it, forming deep reservoirs 

 of water, deep springs, constant or intermittent or when 

 tapped deep wells are formed. This deep water may also 

 pass into a river, lake, or into the sea, its movement being 

 fairly constant but slow. The level of this underground water 

 varies much according to the rainfall, adjacent springs, river 

 level, &c. 



Subterranean water is then classified according to its relation tO' 

 the first impermeable stratum thus : — 



{A) Ground Water above the impermeable layer forming : — 

 (i) Shallow wells, 

 (2) Surface springs. 



(jB) Deep-seated TTV/ier beneath the impermeable layer forming: — 

 (i) Deep wells. 



(2) Nearly all springs, 



(3) Artesian wells; in these the water must be under sufficient 

 hydrostatic pressure, to force itself to the surface when the 

 well is bored. 



Shallow Wells and Springs. 



The depth varies from 2 to 50 feet according to the depth of the 

 upper impervious stratum and the depth of the water lying upon it. 



The area drained and affected by pumping from it varies according 

 to the nature and lay of the surrounding soil, but it may be from 

 20 to 200 times the area of the depression. 



The water mav be pure while the drainage area is free from- 

 contamination as was the case in the South African War before men 

 and horses were picketed upon it, and as at Maidstone before the 

 hop-pickers squatted upon it, and as in certain districts of India prior 

 to their occupation by pilgrims. 



