The regulators in use at Worms and those recently introduced at Magdeburg act upon the same principle, but the levels of the water on the filters are allowed to fluctuate, and the weirs and in fact, the whole regulating appliances are mounted on big floats in surrounding chambers of water connecting with the unfiltered water on the filters. I am unable to find any advantages in these appliances, and they are much more complicated than the forms shown by the cuts. APPARATUS FOR REGULATING THE RATE DIRECTLY. [Illustration: FIG. 9.—LINDLEY’S REGULATION APPARATUS AT WARSAW, RUSSIA.] The above-mentioned regulators control directly the loss of head, and only indirectly the rate of filtration. The regulators at Warsaw were designed by Lindley to regulate the rate directly and make it independent of the loss of head. The quantity of water flowing away is regulated by a float upon the water in the effluent chamber, which holds the top of the telescope outlet-pipe a constant distance below the surface and so secures a constant rate. As the friction of the filter increases the float sinks with the water until it reaches bottom, when the filter must be scraped. A counter-weight reduces the weight on the float, and at the same time allows a change in the rate when desired. This apparatus is automatic. All of the other forms described require to be occasionally adjusted by the attendant, but the attention they require is very slight, and watchmen are always on duty at large plants, who can easily watch the regulators. The Warsaw apparatus is reported to work very satisfactorily, no trouble being experienced either by leaking or sticking of the telescope-joint, which is obviously the weakest point of the device, but fortunately a perfectly tight joint is not essential to the success of the apparatus. Regulators acting upon the same principle have recently been installed at Zürich, where they are operating successfully. Burton[16] has described an ingenious device designed by him for the filters at Tokyo, Japan. It consists of a double acting valve of gun metal (similar to that shown by Fig. 11), through which the effluent must pass. This valve is opened and closed by a rod connecting with a piston in a cylinder, the opposite sides of which connect with the effluent pipe above and below a point where the latter is partially closed, so that the valve is opened and closed according as the loss of head in passing this obstruction is below or above the amount corresponding to the desired rate of filtration. The use of the Venturi meter in connection with the regulation of filters would make an interesting study, and has, I believe, never been considered. [Illustration: REGULATOR-HOUSE, SHOWING RATE OF FILTRATION AND LOSS OF HEAD ON THE OUTSIDE, BREMEN.] [Illustration: INLET FOR ADMISSION OF RAW WATER TO A FILTER, EAST LONDON. [_To face page 58._] ] APPARATUS FOR REGULATING THE HEIGHT OF WATER UPON FILTERS.