134 



TRANSACTIONS 



i899-'oo 



to the level B. This last is the so-called ground-water level ; and 

 it is evident that the depth of the ground- water level, below the 

 surface level, will vary with the rainfall, and with the thickness 

 of the stratum S : will be highest after heavy rain, and lowest 

 after prolonged drought. 



For every region an average may be struck, which is known 

 as the mean annual ground- water level, and we may suppose B 

 to represent this mean level for the area under consideration. If 

 now, a well be sunk, as at W, to the clay, this well will contain 

 water to the depth of the average ground water of the locality. 

 If the well be carried lower, i.e., into the clay, the result will 

 not be to change the character of the water, but merely to 

 enlarge the storage capacity of the well. Extensive areas of 

 level land, as in prairie regions, beaver meadows, etc., answer to 

 the conditions described ; but certain other characteristics of 

 ground-water must be considered. 



Ground- water is never stagnant, but is moving more or less 

 rapidly towards some line of lower level, where a brook, or river, 

 or lake will generally be found. In Fig. 2, D represents a 

 section of a stream, whose waters will evidently rise and fall 

 with the level of the ground water, which supplies them ; and 

 we must discriminate between this rise and fall, which is always 

 gradual, and that sudden rise, of short duration, which results 

 ^r6iri'^the"surface flow immediately following heavy rain. 



B 



Fig. 2. 



The flow of the ground-water will be in the direction 



indicated by the arrows, i.e., at right angles to the course of the 



stream, and the line B B, will be a gradually sloping one, so 



that the well W^ will contain less water than W2 at the same 



