GENERAL CONSIDERATION. 3' 



rocks, which can only act as permeable masses when they are excep- 

 tionally fissured. 



These underground reservoirs formed by the water contained 

 in porous rocks may be of two sorts. The 



Ground-water. 



upper surface of the underground reservoir may 

 be in direct communication with the atmosphere through the pores 

 of the superincumbent unsaturated rock. The reservoir is then said 

 to consist of "ground-water" or "subsoil-water." If a shaft be 

 sunk so as to penetrate the soil deeper than the surface level of this 

 underground reservoir, water will percolate and fill the cavity up to 

 that level. These are ordinary percolation wells or surface wells. 

 But the underground water is not always in direct communica- 

 tion with the atmosphere immediately above it. 



Artesian conditions. 



We know that the composition of that part 

 of the earth nearest the surface, the " earth's crust," which is acces- 

 sible to observation, is far from being homogeneous. The rock 

 nearest the surface at one particular point may be of a highly im- 

 permeable nature, and it may extend to a depth far greater than 

 that of the normal level of permanent saturation. Supposing there 

 be a porous layer beneath this impermeable one, it cannot, however 

 highly porous, receive water from the surface directly above it, 

 because of the intervening stratum opposing itself to percolation 

 from above. Yet, if this porous layer extends laterally to a greater 

 distance than the overlying impermeable one, it must come into 

 communication with the surface at some other more or less remote 

 point. In this way it can obtain a supply of water, and if, at the 

 place where it receives the supply, the permanent level of satura- 

 tion stands at a higher altitude than the bottom of the impermeable 

 rock that overlies it elsewhere, the porous layer may be enclosed 

 by impermeable rocks in such a way that the water, at the place 

 first considered, not only exerts a pressure downwards but also 

 presses upwards on the lower surface of the bed above it. If a shaft 

 be sunk at that place, the water will rise to a point higher than 



( 5 ) 



