and Subterranean Water Storage. 75 



are conceivable. It might remain at rest beneath the partial or 

 absolute surface of saturation, having found its natural level, 

 exposed to reduction only from evaporation taking place in 

 the rock above it : it would then be in the condition of water 

 in a tank. But it might circulate between two impermeable 

 strata, and run between them from one point underground to 

 another, and in this case it must act as if confined in a tube. 

 In the former case, the strata above can never be absolutely dry, 

 because if there be any M^ater whatever in an absorbent rock, 

 a part at least is distributed through the mass. This I have 

 myself proved by actual experiment in the case of chalk.* It is 

 also clearly indicated by the vegetable growth that takes place 

 on the surface of limestone, apparently quite dry during the 

 whole of a perfectly rainless summer in hot countries. This also 

 is a fact within my own personal experience. Whenever water 

 is present in a rock, it is distributed through it, but there is more 

 below than above. The distribution takes place by capillary 

 action, and cannot be checked, but the reservoir is in the lower 

 part of the rock. If then into such rock or rocks so situated, 

 having even at great depth a surface of absolute saturation, we 

 penetrate to such surface, we shall certainly reach water. This 

 is the case of land sprinf/s, commonly so called. It differs 

 mechanically from the case of Artesian springs. 



Although water percolates with a certain amount of freedom 

 through rocks of all kinds, there must always be a great deal of 

 friction in the operation, so that time is needed to effect the dis- 

 tribution, even under the most favourable circumstances. Thus, in 

 the case of land springs of the ordinary kind in sands and gravels, 

 the effect of a single shower may perhaps be recognised within 

 four-and-twenty hours. Where the gathering ground is larger, 

 and the deposit thicker, a wet season may actually leave the 

 springs lower than before, while a subsequent dry season may be 

 followed or accompanied by an unusual flow of water. This 

 must happen when the position of the surface of saturation is 

 so far removed from immediate influence as to need months or a 

 year or two to convey the water from the surface of the ground. 



But if absorbent or loose strata act as tubes or pipes, the 

 water contained in them will be forced on by a pressure corre- 

 sponding to that of a vertical column of water, equal in height 

 to the depth from the surface of permanent wetness, or that at 

 which the impermeable beds are entered, and where, therefore, 

 the pipe condition begins. It will not be equal or nearly equal 

 to" the full calculated result of such pressure, because of the 

 friction, which greatly diminishes the force, but still the pressure 



* See Excerpt 'Minutes of Transactions of Soc. of Ci\il Engineers.' 



