WATER 3 



and amount of such solids. The water as brought into the basin 

 by tributary streams may hold much matter in suspension, especially 

 after heavy rain, but the greater part soon becomes deposited. 



Reservoirs are made when it is desirable to increase the water 

 supply of towns, and to ensure that there shall be no shortage in 

 times of drought. They are simply artiiicial lakes made by dam- 

 ming up natural outlets of catchment areas. The water from 

 reservoirs is similar to that obtained from natural lakes. 



So far then we have considered what may be described as surface 

 waters, though these may have contributions from sub-soil collec- 

 tions. The fate of the water after it sinks into the soil depends 

 upon the character of the ground. Water, as it percolates through 

 the pervious soil, takes up the carbonic acid that is present, which 

 imparts to it the power to dissolve calcium carbonate, magnesium 

 carbonate, potassium carbonate and sodium carbonate, and to a 

 certain extent to act upon the silicates. 



In addition to the mineral taken up, the water gathers a variable 

 quantity of organic matter; this may be of a harmless nature or 

 consist partly of excretal products from people and animals. All 

 surface water, therefore, must be regarded with suspicion. 



Wells are shafts or tubes sunk into the soil. They are of three 

 kinds, shallow, deep and artesian. 



A shallow well is a shaft sunk into pervious soil, and which 

 collects water from close proximity to it. The term " shallow " 

 gives no indication of the depth of the well, a shallow well may 

 be 6 or 60 feet deep, though it rarely exceeds 40 or 50 feet. So 

 long as the shaft does not penetrate an impermeable stratum, such 

 as clay, it remains a shallow well, whatever its depth may be. 



A deep well is a shaft sunk through, first, a pervious layer, 

 and second, an impervious layer, and draws water only from below 

 the impermeable stratum. Water which has reached this position 

 has passed through the surface soil some considerable distance 

 away, and has thus had sufficient time and opportunity to become 

 purified by natural filtration. It must be clearly understood that 

 surface water must be kept out of such a well for it to be a true 

 deep well. This is done by lining the interior of the well with 

 bricks faced with cement. This protection against the inlet of 

 surface water, which is called steining, should in theory reach right 

 down to the impermeable stratum, but in many cases it ceases 

 before this depth is reached. The depth to which the steining is 

 taken depends largely upon the nature of the soil and the pollution 

 that is likely to take place. 



Deep well water is pure if the well is properly constructed. 



