Such a construction is peculiarly applicable to the purification of water, either alone, or combined with the downwards plan of filtration. Of the former variety an example is shown in [fig. 391.] The wooden or zinc conical vessel is provided with two perforated bottoms or sieves e e, betwixt which the filtering substance is packed. Over this, for the formation of the space h h, there is a third shelf, with a hole in its middle, through which the tube d b is passed, so as to be water tight. This places the upper open part of the apparatus in communication with the lowest space a. From the compartment h h a small air tube l runs upwards. The filtering substance consists at bottom of pebbles, in the middle of gravel, and at the top of fine sand, which may be mixed with coarsely ground bone-black, or covered with a layer of the same. The water to be filtered being poured into the cistern at top, fills through the tube b d the inferior compartment a, from which the hydrostatic pressure forces the water upward through the perforated shelf, and the filtering materials. The pure water collects in the space h h, while the air escapes by the small tube l, as the liquid enters. The stopcock i serves to draw off the filtered water. As the motion of the fluid in the filter is slow, the particles suspended in it have time to subside by their own gravity; hence there collects over the upper shelf at d, as well as over the under one at a, a precipitate or deposit which may be washed out of the latter cavity by means of the stopcock m.
As an example of an upwards and downwards filter, [fig. 392.] may be exhibited. A B C D is a wooden or metallic cistern furnished with the perforated shelf c d near its under part, upon which a vertical partition is fixed through the axis of the vessel. A semicircular perforated shelf is placed at a, and a second similar one at b. These horizontal shelves rest upon brackets in the sides of the cisterns, so that they may be readily lifted out. The space G is filled with coarse sand, J with moderately fine, and H with very fine. The foul water is poured into the chamber E, and presses through G J H and into the space F; whence it may be drawn by the stopcock f.
[Fig. 393.] represents in section a filtering apparatus consisting of two concentric chambers; the interior being destined for downwards filtration, and the exterior for upwards. Within the larger cistern A, a smaller one B is placed concentrically, with its under part, and is left open from distance to distance, to make a communication between the interior cavity and the exterior annular space. These cavities are filled to the marked height with sand and gravel. The inner cylindrical space has fine sand below, then sharper sand with granular charcoal, next coarse sand, and lastly gravel. The annular space has in like manner fine sand below. The foul water is introduced by the pipe E, the orifice at whose end is acted upon by a ball-cock with its lever a; whereby the water is kept always at the same level in the inner vessel. The water sinks through the sand strata of the middle vessel, passes outwards at its bottom into the annular space, thence up through the sand in it, and collecting above it, is let off by the stopcock on the pipe b. When a muddy deposit forms after some time, it may be easily cleared out. The cord e, running over the pulleys f f, being drawn tight, the ball lever will shut up the valve. The stopcock d made fast to the conducting tube E must then be opened, so that the water now overflows into the annular space at A; the tube c, in communication with the inner space B, being opened by taking out the stopper h. The water thereby percolates through the sand strata in the reverse direction of its usual course, so as to clear away the impurities in the space B, and to discharge them by the pipe c h. An apparatus of this kind of moderate size is capable of filtering a great body of water. It should be constructed for that purpose of masonry; but upon a small scale it may be made of stone-ware.
