56 TRANSACTIONS OF THE [nOV. 29, 



filter is operated without starting the reverse current, the greater 

 the depth to which the dirt penetrates. As a general rule, 

 however, the dirt is retained within the first six inches, unless 

 the filter is run for longer than twenty-four hours without 

 cleansing, and since the upper pipes for cleaning are placed a 

 foot below the surface of the bed, the rapid washing of this foot 

 of sand when the reverse current is sent through it cleanses 

 the filter thoroughly. The tendency is for the water to gradually 

 form channels through the entire body of the filter-bed. For 

 this reason, the bed is mechanically made over again and, as it 

 were, renewed, by calling into play the lower series of double 

 perforated pipes. After first cleansing the surface, in the man- 

 ner just described, by the surface pipes, the reverse current is 

 made to pass through the lower pipes. Every portion of the 

 bed is floated loose, and by the attrition of one particle of sand 

 against another the impurities are scrubbed off, floated to the 

 top, and carried off through the valve B. When the cleansing 

 is complete, which is shown by the pipe B delivering bright, 

 clear water, the reverse current is shut off, and the filtering 

 material settles down once more into a new uniform filter-bed. 



The arrangements for aerating and filtering are shown in the 

 annexed cut. The three filters are each 10 feet in diameter, 

 and each capable of filtering 350,000 gallons per diem. 425 

 bushels of filtering material are required for each filter. The 

 water passes in through the pipe D, and the filtered water out 

 through the pipe Gr. The air is driven in under the amount 

 of pressure required, and in the quantity requisite to bring 

 the percentage of dissolved oxygen to a maximum, through the 

 pipe C. The air- compressor is represented at the left of the 

 drawing, and maybe made to deliver either a very large amount 

 or a very small amount of air, as the water requires. The 

 mechanical arrangements for adding the precipitants, which are 

 required in some cases, are not shown. The precipitants are 

 usually added before the introduction of the compressed air, 

 though sometimes after. The nature and amount of the pre- 

 cipitant, and the quantity of air required, is determined in each 

 case by the character of the water as revealed by analysis. If it 

 is a water whose only impurity is suspended matters, the filters 

 alone are requisite. If, in addition to this, it is foul-smelling and 

 tasting, and has lost the proportion of oxygen normally present 

 in sweet waters, the aerator is attached to the filter. If it is 

 hard, or filled with very minute suspended particles, or dyed 

 with peat extract, etc., the precipitant of lime or alum or 

 both is added. It is subsequently aerated or not, as the charac- 

 ter of the water demands, and then both precipitant and pre- 

 cipitate are removed by filtration. 



