The description runs as follows: "The 'compressed charcoal' filtering-beds have a deodorizing and decolorizing power, removing suspended or mechanical impurities, as well as certain dissolved bodies, and effecting a chemical change in the water filtered". This is all right. To carry on the filtration and purification of large quantities of water, a series of such filters must be employed, and special attention has to be paid to frequent cleansing. In England these "filter-beds" are largely employed for city and town supply.

Fig. 36. - Self-Acting Cistern Filter.