604 THE POPULAR SCIENCE MONTHLY. 



distinct from its capacity for imbibition. If the component parti- 

 cles of a rock — for instance, the quartz pebbles of a loose conglom- 

 erate or the grains of a sandstone — present an impervious surface, 

 water will cohere to the individual surfaces until the entire speci- 

 men is enveloped in a coat of water. If the interstices are smaller 

 than the average drop of water, the resistance of cohesion to the 

 transmission of water will be greater ; hence a chalk or a fine- 

 grained brick will drink in much water, but will transmit it 

 slowly, while water will pass rapidly through coarse gravel. The 

 capacity for transmission in variously grained rocks and the 

 accompanying cohesion is similar to that seen in passing water 

 through sieves of different mesh. Thus, some sandstones of ex- 

 actly the same capacity for imbibition as chalk transmit water 

 six hundred times faster. 



The rock materials of the earth with these different capacities 

 for imbibition and transmission have been sorted into definite 

 sheets or strata by the water which deposited them, and thus 

 another important fact in the question of underground water is 

 introduced — the stratification or arrangement of the rocks relative 

 to one another. Earth water percolates downward through a 



Fig. 5. — Unfavorable Conditions fob Aktesian Water. 



porous stratum until an impervious one is reached, while an im- 

 pervious stratum at the surface will prevent the saturation of a 

 pervious one below. Stratification performs the important func- 

 tion of controlling the distribution of earth water, of resistance, 

 transmission, and storage. If the surface rock stratum is per- 

 vious and horizontal, it simply serves as a sponge to hold the 

 water until disturbed by evaporation or seepage, unless the sup- 

 ply is constantly renewed by rainfall. (See Fig. 3.) 



If an impervious sheet is above an inclined outcropping porous 

 stratum (Fig. 4), it opposes the tendency of water to rise by hy- 

 drostatic pressure and retains it in the porous sheet. If an im- 

 pervious stratum is beneath a porous one, it prevents the water of 

 the latter from percolating to greater depths. If vertically ar- 

 ranged from folding, the including strata cut off the horizontal 

 transmission of underground water. (Fig. 5.) 



Water is transmitted by gravity in the direction of the in- 

 clination of the strata — i. e., with the dip ; and if the topographic 

 conditions are favorable, flowing wells can be obtained at lower 

 points more or less distant from the outcrop. If the strata in- 



