4 VETEEINAEY HYGIENE 



the earth vpas free from any inorganic matter, what is 

 commonly known as ' soft,' it now becomes ' hard ' in eon- 

 sequence of the amount of Hme, magnesia, etc., it has 

 taken up. 



In its passage through the various layers forming the 

 crust of the earth, the ground water, as it is called, at last 

 meets a layer of impervious rock through which it cannot 

 pass, excepting through such cracks or joints as the rock 

 may contain. This impervious layer causes the flow of 

 water to turn aside, and seek by the action of gravity an 

 escape through a neighbouring pervious layer. 



This is the principle which governs the production of all 

 springs, wells, and borings. The water collecting area of a 

 boring may be many miles away, in the case of the great 

 well at Artois, from which the term artesian is derived, the 

 water enters the earth sixty miles away. 



Springs are simply the outlet of the underground water, 

 and the yield from them depends upon the rainfall on the 

 collecting area. Some springs are known to originate at a 

 considerable depth, perhaps some miles, in the earth. 

 These are designated deep springs, their water is often at 

 a high temperature, and so charged with mineral matter 

 as to be useless for drinking purposes, though of medicinal 

 value. 



In the case of London a pervious layer of chalk exists at 

 some depth below the city, which comes to the surface 

 many miles away in the hills found to the north, west and 

 south of the London basin. Over this pervious layer is a 

 bed of clay known as London Clay, quite impervious, and 

 to reach the water in the chalk this clay must be bored 

 through. 



Springs may be a considerable source of supply to rivers. 

 There are springs in the bed of the Thames near Windsor 

 calculated to deliver 300,000,000 gallons of water daily into 

 the river. 



The ordinary summer rainfall does not undergo percola- 

 tion, but owing to the temperature of the air it is removed 

 by the process of evaporation, both direct evaporation 



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