only exit is through the cloth or paper, the solid portion is
kept back while the liquid passes through and escapes by the drainage
surfaces to the outlets. The cells are subjected to pressure, which
increases as the operation goes on, from the growing resistance offered
by the increasing deposit of solid matter on the cloths; and it is
therefore necessary that they should be provided with a jointing strip
around the outside, and be pressed together sufficiently to prevent any
escape of liquid. In ordinary working both sides of the cell are exposed
to the same pressure, but in some cases the feed passages become choked,
and destroy the equilibrium. This, in the earlier machines, gave rise to
considerable annoyance, as the diaphragms, being thin, readily collapsed
at even moderate pressures; but recently all trouble on this head has
been obviated by introducing the three projections near the center, as
shown in the cuts, which bear upon each other and form a series of stays
from one end of the cells to the other, supporting the plates until the
obstruction is forced away. We give an illustration below showing the
arrangement of a pair of filter presses with pneumatic pressure
apparatus, which has been successfully applied for dealing with sludge
containing a large amount of fibrous matter and rubbish, which could not
be conveniently treated with by pumps in the ordinary way. The sludge is
allowed to gravitate into wrought iron receivers placed below the floor,
and of sufficient size to receive one charge. From these vessels it is
forced into the presses by means of air compressed to from 100 lb. to
120 lb. per square inch, the air being supplied by the horizontal pump
shown in the engraving. The press is thus almost instantaneously filled,
and the whole operation is completed in about an hour, the result being
a hard pressed cake containing about 45 per cent. of water, which can be
easily handled and disposed of as required. The same arrangement is in
use for dealing with sewage sludge, and the advantages of the compressed
air system over the ordinary pumps, as well as the ready and cleanly
method of separating the liquid, will probably commend itself to many of
our readers. We understand that from careful experiments on a large
scale, extending over a period of two years, the cost of filtration,
including all expenses, has been found to be not more than about 6d. per
ton of wet sludge. A number of specimens of waste liquor