130 



SMALL WATER SUPPLIES. 



We now come to the important case of a reservoir, 

 on the side of a hill, to store spring water, as in fig. 93. 



Our first calculation is that of the earth pressure 

 on the right-hand wall, which in this case is sur- 

 charged, that is, the earth has a sloping surface up- 

 wards from the wall. Set up the line AB at the 

 same angle from the vertical as the natural slope of 

 the earth, and produce the slope of surcharge to 

 meet it. Bisect AB and describe the semicircle. 

 Draw xy perpendicular to AB, cutting the semicircle 



FIG. 93. 



in y. Now the horizontal thrust at one-third the 

 height of the wall will be 



. (43) 



The weight of the wall will be found to be 25 cwt. 

 In the manner shown, this must be combined with the 

 thrust, and the resultant drawn. The distance. from 

 the point where the resultant cuts the base of the 

 wall from the centre must then be accurately mea- 

 sured, the diagram being done to a large scale, say 

 it is 71 feet. 



