2l6 THE WATER PROBLEM OF A LARGE CITY 



Small elevated tanks, like those of the windmill, frequently 

 have heavy iron bands around their lower portion as a pro- 

 tection against the extra strain. 



Before erecting a dam or reservoir, the maximum pressure 

 to be exerted upon every square inch of surface should be 



accurately calculated, and 

 the structure should then be 

 built in such a way that the 

 varying pressure of the 

 water can be sustained. It 

 is not sufficient that the 

 bottom be strong ; the sides 

 likewise must support their 

 strain, and hence must be 

 FIG. 156. The lock gates must be strong in increased in strength with 



order to withstand the great pressure of the . , 



water against them. depth. This strengthening 



of the walls is seen clearly 



in the reservoir shown in Figure 152. The bursting of dams 

 and reservoirs has occasioned the loss of so many lives, and the 

 destruction of so much property, that some states are consider- 

 ing the advisability of federal inspection of all such structures. 



200. The Relation of Forests to the Water Supply. When 

 heavy rains fall on a bare slope, or when snow melts on a 

 barren hillside, a small amount of the water sinks into the 

 ground, but by far the greater part of it runs off quickly and 

 swells brooks and streams, thus causing floods and freshets. 



When, however, rain falls on a wooded slope, the action is 

 reversed ; a small portion runs off, while the greater portion 

 sinks into the soft earth. This is due partly to the fact that 

 the roots of trees by their constant growth keep the soil loose 

 and open, and form channels, as it were, along which the 

 water can easily run. It is due also to the presence on the 

 ground of decaying leaves and twigs, or humus. The decay- 



