STORAGE AND DISTRIBUTION. 



133 



When a tank is closed provision should always be 

 made for the admission of air which will tend to keep 

 the water pure. Simple holes may be left in the roof, 

 and short lengths of fireclay pipe with galvanized 

 gratings set in the sockets will suffice, provided they 

 are a few inches above ground level, to prevent any 

 dirt falling into the water. A current of air may be 

 induced by the use of Boyle's patent air-pump venti- 

 lators, upcast and downcast heads being used, but 



12 6 



FIG. 96. 



this, of course, is merely a refinement and not an ab- 

 solute necessity (or see detail, fig. 112 a). Where 

 circumstances allow of such a construction, a very 

 convenient way of storing water which is pumped 

 (the construction also is quite admissible for gravi- 

 tation supplies) is by means of an elevated tank, 

 which also forms the roof of the engine-house, and 

 is supported on the walls of the same. Referring to 

 figs. 97 and 98, we have a rectangular tank 15 ft. 

 square, and holding water to a depth of 4 ft. 6 in. 

 Its construction, by means of reinforced concrete, is 



