PROPERTIES OF WATER AND SOURCES OF SUPPLY. I 5 



and stored, but in the meantime it is necessary to 

 pass on to the much more important question of 

 well water and spring water. We have seen that 

 of the rainfall part of it goes into the rivers. 

 Some also evaporates in quantities which con- 

 cern the engineer of large waterworks. A small 

 quantity is absorbed by plant life, especially in 

 early summer, while a large portion sinks through 

 the soil and goes to form the great body of sub- 

 terranean water which has been referred to, and 

 from which our springs derive their supply. The 

 proportion of the rainfall yielded by springs has 

 attracted the attention of waterworks engineers for 

 some time, a quantity varying with the geological 



FIG. 2. 



structure, clay being very impermeable and gravel 

 permeable, the latter being conducive to the forma- 

 tion of springs, as is also a level tract of country in 

 preference to a hilly district. First of all we have 

 surface springs, where a tract of gravel of the shape 

 of a flat dome lies on top of a bed of clay ; all along 

 the lower ridge of the gravel, where the clay comes 

 to the surface (or outcrops), springs may be capable 

 of formation owing to the water being prevented 

 from passing through the clay. The idea is shown 

 by fig. 2, and the basin-shaped mass of gravel lying 

 on the clay will always be in a state of saturation. 

 This is a very commonly occurring instance of a 

 spring, but the water is not of as good a quality as 

 that obtained from deep wells. Again, a fault in the 



