STORAGE AND DISTRIBUTION. 



143 



surface, it will be found that the tank will not be too 

 large and yet not allow much to pass away unused, 

 but it may be much increased if water is scarce and 

 every available drop is required. 



Another system is shown in fig. 107. It will be 

 seen that the filter is not covered. This is ad- 

 visible as light and air help to purify the water 

 during filtration, while the fine sand on top will want 

 periodically scraping, which is not easy to do in a 

 closed tank unless the depth between roof and the, 

 top of the sand admits of a man standing up. After 

 a time the sand will be found to get a jelly-like 



w.o -> 





FIG. 107. 



slime on the top, which impedes the working of the 

 filter and requires removal. It has a certain valufe 

 in the purification of water and is a matter to which 

 waterworks engineers attach much importance, but 

 its discussion would be out of place here, and the 

 reader is referred to the larger works on water 

 supply. The efficiency of such a filter depends 

 largely upon the ratio of filtration, 3 in. per hour 

 being quite fast enough. An overflow pipe con- 

 nected from the top of the tank to a drain would 

 pass away any surplus water that could not be dealt 



