602 THE POPULAR SCIENCE MONTHLY. 



certain deep-seated artesian wells the source is always the rain 

 which falls in the immediate vicinity, as the physician knows 

 when called to treat disease caused by seepage of the adjacent 

 surroundings into the family well. 



Part of the rainfall is quickly drained away by the surface 

 channels, a part is evaporated, and a third, and for our considera- 

 tion the most important part, is imbibed by the rocks and soil. 

 The proportional disposition of the rainfall in the above manner 

 varies with the climate and geologic conditions, but so far as 

 underground waters are concerned it is necessary to consider only 

 the water which sinks into the ground. 



That portion of the earth visible to human inspection, known 

 as the crust, is more or less saturated with water. In times of 

 drought and in the arid region this is not always evident at the 

 immediate surface, where evaporation is taking place, but a post 

 hole, a plow furrow, a blast in a quarry, or a newly dug well re- 

 veals the dampness of the rock material. This moisture is some- 

 times invisible to the eye, but in general its quantity varies in 

 proportion to the compactness or porosity of the rocks, the num- 

 ber of joints, fissures, or other crevices, and the topographic situ- 

 ation which controls the drainage. 



If rainfall be long continued, the portion of the crust upon 

 which it falls becomes completely saturated. Upon cessation of 

 the rain, evaporation or drying begins at the surface, causing the 

 line of saturation to sink deeper and deeper. Thus it is that in 

 the Eastern States, where rainfall is excessive and evaporation 

 slow, the line of saturation usually coincides with the surface, 

 while in the arid regions it is often several hundred feet below. 

 In this section, holes three hundred feet deep are often drilled 

 through soil and rock as dry as powder without reaching the line 

 of saturation, while on the East, as for example in New Orleans, 

 water is so near the surface that dry graves can not be dug for the 

 dead. 



If the earth were of uniform porosity, temperature, and com- 

 position the water it contains would be uniformly distributed 

 through it, as is the water in a well -soaked sponge. But this is 

 not the case, for the outer portion of the globe consists of rocks of 

 much less density than are those of the interior, while the down- 

 ward percolation of water in some instances encounters the super- 

 heated mass of the earth's interior, and is forced back to the sur- 

 face as steam, as in geysers and volcanoes, or enters into mineral 

 combinations. Hence the available water is confined to that por- 

 tion of the earth's crust between the lines of heated interior and 

 surface evaporation. Even in this narrow belt the distribution 

 of water is very irregular. 



Inasmuch as there is a great diversity of geologic structure, 



