8 VETEEINARY HYGIENE 



always flowing in the direction of its outlet, and if a well 

 has been sunk so as to catch this ground-water, it also 

 catches the fluid parts of the cesspit. 



Further, the greater the demand on a well the larger the 

 area drained by it, so that though the well may be placed 

 on ground above the cesspit, and thus under ordinary 

 conditions moderately safe, yet when a great demand is 

 made upon it, the fluid in the cesspit below may con- 

 tribute. 



A well drains an extent of ground represented by an 

 inverted cone, the radius of which is at least four times its 

 depth, sometimes even more, as when the drain on the well 

 is considerable. 



In choosing a site for a well, the distance of any possible 

 polluting source should be from 100 to 160 times the depth 

 measured to the surface of the water, even greater if the 

 lowering of the water in the well is likely to be consider- 

 able. 



Every care should be taken to exclude surface-water 

 from passing into wells. Much may be done by making 

 the shaft water-tight by brickwork and a cement lining. 

 In the case of a shallow well this should be carried down to 

 the level of the water. The well should be protected by a 

 cover, and a coping to prevent surface-water finding its way 

 in ; the water should be drawn by a pump. 



Artesian wells are of great depth, the water supplying 

 them is generally confined in a reservoir between two 

 impervious rocks ; it finds its way there through a pervious 

 stratum which comes out on the surface of the ground, 

 perhaps some considerable distance away. When the 

 reservoir is tapped, the water at once rises to regain its 

 level. 



The water from deep wells is generally pure, that from 

 artesian is organically very pure, though at the same time 

 it may be very hard. Shallow well-water must always be 

 regarded with suspicion. 



The following Classification of Waters is given by the 

 Kivers Pollution Commissioners : 



Digitized by Microsoft® 



