At Bremen and at Altona a different procedure has been adopted. The filters are all upon the same level, and of the same construction. When a filter is put in service the effluent from it, instead of being taken to the pure-water reservoir, is taken to another filter which has already been some time in service. After the first filter has been in operation for some time its effluent is taken to the pure-water reservoir, and in turn it is supplied with the effluent from a filter more recently cleaned. The loss of head of water passing a freshly cleaned filter is comparatively slight, and the water of the second filter is allowed to fall a few inches below the high-water mark, at which level it will take the effluent from the other filter. The connections between the filters are made by siphons of large pipe, the summits of which are considerably above the high-water line. These siphons are filled by exhausting the air, and when opened to the air there is no possibility of a flow of water through them. The process has given extremely good results in practice, yielding effluents of the very greatest purity and at a quite moderate cost of operation. An objection to the method is the possible filling of a siphon some time when the water standing upon the after-filter is higher than that in the pure-water well of the fore-filter, and while the fore-filter is connected with the pure-water reservoir. Such a connection would send unfiltered water into the pure-water reservoir direct. I do not know that any trouble of this kind has ever been experienced at Bremen or at Altona; and the objection to this system is perhaps not well founded where the management is careful and conscientious. The fact that an unscrupulous attendant can make the connection at any time to help out a deficiency of supply, or simply through carelessness, is certainly objectionable. For the treatment of river-waters and lake-waters containing only a small quantity of sediment, and where the removal of bacteria or disease-producing qualities is the most important object of filtration, sand filters can be used. Where the rivers are subject to floods and moderate amounts of muddy water, sedimentation-basins or storage reservoirs for raw water will often be found advantageous. For the treatment of extremely muddy waters, and waters which are continuously muddy for long periods of time, and for the removal of color from very highly colored waters, resource must be had to coagulants. The coagulants which are necessary in each special case and which can be used without injury to the water must be determined by most careful investigation of the raw water.