FIELD GEOLOGY. 219 



The subject may "be advantageously divided into 

 three sections : 



1. Origin of the supply. 



2. Streams and springs. (Natural founts.) 



3. Wells. (Artificial founts.) 



1. Origin of the Supply. The original source whence 

 is derived all our supply of water whether it come to 

 our hand through natural or artificial founts is, of 

 course, the " Eainfall." This varies every year and in 

 every locality, but is always and everywhere (with rare 

 exceptions) much in excess of what is actually neces- 

 sary to the population. Then, it may be asked, " how 

 is it that our supply ever seems likely to be unequal 

 to the demand ?" The answer is simple. We do not 

 economise that derived from the natural founts, and we 

 do not sufficiently avail ourselves of the artificial founts, 

 which are at all seasons practically inexhaustible. The 

 following statistics, concerning England, will help to 

 elucidate the matter. 



Thirty gallons per day or 50 tons per year is the 

 average quantity required for each head of the 

 population. One inch of rainfall to one acre is 

 equal to 100 tons. There are 1J acres to each 

 head of population. 



Therefore a quantity equal to a rainfall of only one- 

 third of an inch is sufficient for the requirements of 

 the population. 



The minimum Eainfall is about 27 J inches. The 

 average Eainfall is about 34 inches. In dry 

 years (in some localities) it has been but 16 

 inches. In wet years (in other localities) it has 

 been even so high as 20 feet. This excessive 



