UNDERGROUND WATER 149 
the earth, constantly becoming deeper (Fig. 75). A 
well, or a thousand wells, bored down to this, will find 
water which rises nearly to the height of the place 
where the layer first enters the ground. 
So the conditions necessary for an artesian well are 
four: (1) there must be a porous stratum; (2) this 
must be overlaid and underlaid by impervious layers ; 
Occurrence of artesian water where the rocks dip into the earth. S, porous 
sandy layer. J, J, impervious strata. A, A, A, artesian wells. 
(3) these must descend into the earth; and (4) the 
place where the well is bored must be lower than that 
.where the porous layer crops out at the surface, and 
gets its supply of water. Several of the cities of east- 
ern Texas obtain their water supplies from wells which 
reach down to such layers, many miles from the place 
where the water enters them. The same is true also 
in other parts of the West where artesian water is 
used in irrigation, 
