Its operation and description are given as follows:

"The water to be filtered enters at the top and right of filter, at A, as shown in Figure 12, and passes down through the bed of fine sharp sea sand (or coke and sand mixed), and out through the pipe valves G, at the bottom of the filter. These valves are so arranged as to allow the filtered water to pass freely, but will not permit any of the filtering material to escape with the filtered water.

"H is the precipitating device, which can be opened or closed at will, and is arranged to give a certain amount of the alum or other chemical used to the water as it flows into the filter, without obstructing its pressure or flow, and can be closed entirely when washing the filter. In its operation the chemical used to precipitate sewage, vegetable stain, etc., is deposited with the impurities at the top of the bed and thrown out when the filter is washed, no trace of the chemical being found in the filtered water.

"The filter is cleansed or washed by first closing the inlet valve A, then opening the waste valve, B, at the top and left of the filter, also opening the valve, C, to the washing pipe, F, shown in the cut (under the top of the bed of filtering material), which sends a reverse current through the top or surface of the filter bed. Five minutes' time will wash out all the filth and impurities taken from the water during five hours, when the water being filtered is very bad, and it will not be necessary to wash the filter but once a day, unless the water is very muddy and impure.

"It is a well-known fact that in filter beds the impurities taken from the water are all lodged in the first one or two inches at the top of the bed, and that in pressure filters the impurities are retained in the six inches below the top of the bed (unless the filter is run longer than 24 hours without cleansing). In the National Filter the first layer of washing pipes is located from ten or twelve inches below the top of the bed, thus permitting all impurities to be washed out in five minutes' time by sending a reverse current through the top of the bed, thus violently agitating the sand, and, by the attrition, thoroughly cleansing the bed, the impurities passing off through the waste pipe at the left.

" The ability to clean the filter so quickly does away with the necessity, in most cases, of using alum or other chemicals to produce sparkling water, when the water to be filtered contains fine clay or vegetable stain.

"After the filter has been in use several days it should be washed from the bottom, by sending a reverse current of water through the lower series of pipe valves, shown in the bottom of filter (after first washing the top of the bed), in order to break up the passages made by the water in filtering through the bottom part of the bed.

"Ordinarily once a week will answer to wash the lower part of the bed, and for the top, say, once each day, or oftener if the water is very turbid or impure".

The National Filter is used with or without an air compressor. Air forced into water under pressure makes it the more effective and produces a chemical action, which cannot otherwise be achieved. Indeed this ail pressure completes the purification of the water.

The capacity of the filter depends on its size; arrangements, however, for the purification of any quantity of water for the want of a whole community, can be made.