470 



WATER SUPPLY AND SEWAGE DISPOSAL 



and sand slopes down hill, it is undoubtedly true that the 

 ground-water passes much more rapidly through it than it does 

 through the layers of fine clay above and below the gravel. 

 But this would scarcely be called an underground stream; 

 it certainly is not what most people mean by that term. 



The term "underground stream' 7 may properly be applied 

 only to those comparatively rare cases where the water has 

 dissolved portions of the rock (Art. 466), washing the dissolved 

 portion entirely away and leaving an open channel through 

 which the water flows (Fig. 285). But such conditions are so 

 rarely found that the underground stream is not of any importance 

 when considering the source of water supply for wells. 



553. A Vein of Water/ If to " strike a VEIN OF WATER" 

 does not mean the tapping of an underground stream, what 

 does it mean? In meeting this question we shall also be 

 meeting the question, What is the real difference between a 

 good well with plenty of Water and a poor well yielding but 

 little water? 



In Art. 533 we saw that the relation of the water table to the 

 land surface has been determined by noting the height of 



water in wells. We have 

 also seen that the water in 

 a shallow well will stand at 

 exactly the height of the 

 water table, provided that 

 no water is used from the 

 well. Why should using 

 water from the well affect 

 the height of the water in 



the well? If the well is sufficiently deep so that its bottom is 

 below the level of the water table, why should not the well 

 contain a sufficient supply of water at all times? The answer 

 to these questions is this: Although the well does reach below 

 the water table we cannot expect much of a flow till we reach 

 a "vein of water." But what is a vein of water? It is 

 certain that it is rarely, almost never, an underground stream. 



FIG. 286. A vein of water. 



