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 liquors from factories with the residual matters pressed into cakes, and also of the purified effluents, are exhibited.  These will prove of interest to many, all the more so since in some instances the waste products are converted into materials of value, which, it is stated, will more than repay for the outlay incurred.

[Illustration:  Fig. 3.  Fig 4.]

Another application of the filter press is in the Porter-Clark process of softening water, which is shown in operation.  We may briefly state that the chief object is to precipitate the bicarbonates of lime and magnesia held in solution by the water, and so get rid of what is known as the temporary hardness.  To accomplish this, strong lime water is introduced in a clear state to the water to be softened, the quantity being regulated according to the amount of bicarbonates in solution.  The immediate effect of this is that a proportion of the carbonic acid of the latter combines with the invisible lime of the clear lime water, forming a chalky precipitate, while the loss of this proportion of carbonic acid also reduces the invisible bicarbonates into visible carbonates.  The precipitates thus formed are in the state of an impalpable powder, and in the original Clark process many hours were required for their subsidence in large settling tanks, which had to be in duplicate in order to permit of continuous working.  By Mr. Porter’s process, however, this is obviated by the use of filter presses, through which the chalky water is passed, the precipitate being left behind, while, by means of a special arrangement of cells, the softened and purified water is discharged under pressure to the service tanks.  Large quantities can thus be dealt with, within small space, and in many cases no pumping is required, as the resistance of the filtering medium being small, the ordinary pressure in the main is but little reduced.  One of the apparatus exhibited is designed for use in private mansions, and will soften and filter 750 gallons a day.  In such a case, where it would probably be inconvenient to apply the usual agitating machinery, special arrangements have been made by which all the milk of lime for a day’s working is made at one time in a special vessel agitated by hand, on the evening previous to the day on which it is to be used.  Time is thus given for the particles of lime to settle during the night.  The clear lime water is introduced into the mixing vessel by means of a charge of air compressed