TWO POSSIBILITIES 



207 



dividual houses, or to common reservoirs from which it is 

 distributed to the various buildings. 



Figure 148 illustrates in a simple way the manner in which 

 a mountain lake may serve to supply the inhabitants of a 



FIG. 148. The elevated mountain lake serves as a source of water. 



valley. The city of Denver, for example, is surrounded by 

 mountains abounding in streams of pure, clear water ; pipes 

 convey the water from these heights to the city, and thus a 

 cheap and adequate flow is obtained. Such a system is 

 known as the gravity system. The nearer and steeper the 

 elevation, the greater the force with which the water flows 

 through the valley pipes, and hence the stronger the dis- 

 charge from the faucets. 



Relatively few cities and towns are so favorably situated as 

 regards water ; more often the mountains are too distant, or 

 the elevation is too slight, to be of practical value. Cities 

 situated in plains and remote from mountains are obliged to 

 utilize the water of such streams as flow through the land, 

 forcing it to the necessary height by means of pumps. 

 Streams which flow through populated regions are apt to be 

 contaminated, and hence water from them requires public 



