146 SMALL WATER SUPPLIES. 



between them for supporting the roof. These may 



FIG. 112. 



FIG. 1 1 20. 



R.S.J* 



be replaced, if so desired, by cast-iron pillars 



supporting rolled- 

 steel joists upon 

 which jack arches are 

 turned. In fig. 113 

 these jack arches are 

 illustrated, but no in- 

 termediate pillars are 

 required because the 

 joists may safely be 

 used over a span equal 

 FIG. 113- to the distance be- 



tween the side walls. The more modern method of 

 covering such tanks is by means of reinforced con- 

 crete beams and floor slabs, as shown in detail in 

 figs. 1 14 and 1 15. No thrust is then introduced into 

 the walls. Let the beam be 14 in. deep to the 

 centre of reinforcement and 7 in. wide over all. Its 

 span is shown as 7 ft. 6 in. and its weight will be 



