128 



SMALL WATER SUPPLIES. 



built of masonry in the ground for holding water, 

 the sides will, of course, be subject to the thrust of 

 that water. This will generally be greater than any 

 earth pressure likely to come on them. Yet at the 

 same time it might not be the case, especially 

 when the surface of the ground rises rapidly from 

 the wall. Hence the reason for discussing the ques- 

 tion here. Referring to fig. 91, ED is equal to natural 

 slope of ground, and EDF equal to the limiting angle 



A. B 



G. 



FIG. 91. 



of resistance, ABCD equal to the wall. Now BG is 

 made equal to half BE, and the pressure of the earth 

 upon the wall, as we pointed out, is equal to half BG 2 

 multiplied by the weight of i cub. ft. of earth, say 1 10 

 Ib. ; this gives us what is known as the active pres- 

 sure. It is now obvious to the reader that the earth 

 which is behind the angle of repose does not affect 

 the stability of the wall in the least, but the wedge 

 in between is that which takes effect only. By bisect- 

 ing the wedge we have what is termed the line of 



