236 IBBIGATION. 



situated, the whole body of water sinks out of sight, and 

 flows in an underground channel, until it breaks out in 

 copious springs here and there, or in a body at one place. 

 This happens in well known cases in the limestone regions 

 of Kentucky, West Virginia, Florida, and in Texas, 

 where large streams thus suddenly disappear. In other 

 cases considerable streams or lakes pass over or lie upon 

 such porous strata, and a large quantity of water escapes 

 from them. Let it be supposed that, in the diagram 

 given, a stream or lake is situated at the point a, or 

 otherwise that the rainfall of the locality here sinks into 

 the ground and disappears. The water passes through 



Fig. 120. PLAN OP ARTESIAN WELLS. 



the porous stratum, shown by the pebbled shading, (b), un- 

 til the basin is filled. Then if at any point within the basin, 

 c, c, a well be bored until the porous water bearing stratum 

 is penetrated, the water is at once forced to the surface 

 by the pressure, and if confined in a pipe, would rise un- 

 til the level of the source is reached, as shown by the 

 dotted line. This would be an artesian well. 



It is evident that a combination of circumstances, rare- 

 ly existing, must be found to furnish a source of water 

 of this character at all, and that there must be an abun- 

 dant and permanent supply to furnish wells that can 

 yield copiously and permanently. If there is only an 

 accumulated store of rainfall to draw upon, there is 

 danger that it may soon be exhausted, and afterwards 



