The filtering medium employed consists of thick sheets of filtering paper, made of ordinary paper pulp and a quantity of pure animal charcoal, claimed to be free from phosphates by chemical process, added to the pulp before it is formed into sheets. From 10 to 20 per cent, of the weight of this finished paper is said to consist of purified animal char-coal. It will be seen that water to be filtered comes in at one side of the apparatus, and after having passed through the carbon paper is delivered into the service pipe under pressure. When it is desired to change the carbon papers for fresh ones, the filter can be shut off on both sides from the service pipe, and then, by means of a small disc, may be run back, and the grooved plates and distance frames of which the filtering chambers consist can be opened out, and the spent carbon papers changed. When screwed up the machine is again ready for work. These machines are made with from 4 to 12 chambers, and each chamber is provided with a circular disc of the prepared paper on each side, so that a 12-chamber filter, 9 inches diameter, would simultaneously have 24 such paper discs, thus having a large filtering area occupying a small space, in effective work under the pressure of the water main.

These filters for the purposes of greater convenience, and to meet the wants of those desiring it, can be made reversible.

The name "Reversible" is given to those filters which are so arranged that when an accumulation of solid impurities clogs or stops the pores of the filtering medium it can be removed by simply turning a cock, which reverses the current of liquid through the filtering medium, causing all the impurities to flow away through the outlet channel with the flush water, thus effectually cleansing the filter without taking it apart or removing the filtering medium; by returning the cock to its original position filtration proceeds rapidly.

These filters are constructed so that by a simple arrangement any known filtering medium may be applied, be it paper, or paper pulp, woollen, cotton, or other cloth, felt or combinations of any of the above with animal or vegetable charcoal, or any mixture for the purpose of filtration can be introduced into the filter as the filtering medium. The advantage of this will be appreciated, as any filtering medium that may be considered better than another can be applied without any alteration of the apparatus.

Fig. 21. - The Johnson Patent Pressure Filter.