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permeable stratum, and lying upon another impermeable basin- 
shaped deposit, the water then follows the level of the lowest 
point of the permeable stratum just as if flowing on the surface. 
Should an opening be made through the overlying impermeable 
stratum the water rises to a height corresponding with the hydro- 
static pressure, as in the case of Artesian wells. 
Deep seated springs are naturally more constant in the supply 
they yield than land springs. In order to estimate this quantity 
very careful investigation is necessary, and it is a matter of great 
difficulty. 
The next point is to consider the various qualities of waters, 
however derived. 
Of ordinary waters, one varies from another on account of the 
_ different kind and different quantity of foreign matter that each 
contains. This foreign matter may exist in the water, either ina 
' state of mixture or in a state of solution. 
The matter contained in a state of mixture can generally be 
separated by subsidence or filtration. 
Solid matter in a state of solution in any water is either 
mineral or organical. The most common kinds of mineral matters 
present are salts of lime and magnesia, and salts of potash and 
soda. 
It is chiefly the presence of the salts of lime or magnesia that 
causes the water to be termed hard, the presence of the salts of 
soda and potash has no such effect ; the amount of hardness, there- 
fore, depends upon the quantity of lime or magnesia in the water. 
Dr, Clark, some forty years since, proposed to express the 
amount of hardness in any water by the number of grains of lime 
contained in a gallon, thus water containing 20 grains would be 
reckoned at 20 degrees of hardness, and this expression has been 
generally adopted when describing the quality of water. 
Dr. Clark, in further elaborating his experiments, shewed how 
this lime might be removed, and hard water rendered practically 
soft. 
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