4 
STATE GEOLOGIST, 97 
water had settled the bottom away from the side walls for nearly %4 inch. 
Another coat of plaster over the bottom and up the side walls for about 
2 or 4 feet, followed by another wash of neat cement cream, remedied 
the trouble. The cistern was used constantly from that time until 1888, 
when a piped water supply replaced it. For a considerable period of 
time it was in service with 11 foot heads of water in it without any ap- 
preciable loss from leakage. 
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Fig. 40.—Plan of the Reservoir at Aurora, Ill. 
FILTER BEDS. 
It is in underground structures that concrete shows to good ad- 
vantage. Metal work would be subjected to serious corrosion. Brick or 
stone masonry would be either very expensive or else, if a cheaper quality 
of work and material was used, it would not be water-tight and durable. 
In filter beds, clear water basins, reservoirs and the like, impervious, non- 
corrosive materials should be used. Concrete comes nearer meeting all the 
requirements than any other material. For these purposes it has been 
extensively used for many years. | | 
The Philadelphia filter beds illustrate its use for such purposes. The 
Philadelphia water system is divided into several supply stations with 
complete equipment at each station. For the purpose of this article it 
will be sufficient to describe one of the filter beds at the Belmont plant. 
There are eighteen filter beds at this station varying in dimensions from 
166 by 196 feet to 120 by 272 feet, each filter bed covering approximately 
I—S. G. 
