=Adjustment of Gauges.=—The adjustment of the gauges showing the rate of filtration and loss of head is extremely simple. When a filter is put in service the gates from the lower chamber to the pure-water reservoir and to the drain are closed, the outlet of the filter opened, and both chambers allowed to fill to the level of the water on the filter. The length of the wire carrying the gauge is then adjusted so that the gauge will make the desired run without hitting at either end, and then the marker is adjusted. As both the rate of filtration and loss of head are zero under these conditions, it is only necessary to set the markers to read zero on the gauges to adjust them. The gates can then be opened for regular operation, and the readings on the gauges will be correct. [Illustration: INTERIOR OF A FILTER: DRAIN, GRAVEL AND SAND LAYERS.] [Illustration: INTERIOR OF A FILTER, READY FOR USE. [_To face page 306._] ] It is necessary to use wires which are light, flexible, and which will not stretch. At first piano-wire, No. 27 B. & S. gauge, was used, and was well adapted to the purpose, except that it rusted rapidly. Because of the rusting it was found necessary to substitute another wire, and cold-drawn copper wire, No. 24 B. & S. gauge, was used with fair results. Stretching is less serious than it would otherwise be, as the correctness of the adjustment can be observed and corrected readily every time a filter is out of service. From the lower chambers in the regulator-houses the water flows through gates to the pipe system leading to the pure-water reservoir. Drain-pipes are also provided which allow the water to be entirely drawn out of each filter, should that be necessary for any reason, and without interfering with the other filters or with the pure-water reservoir. The outlets of the filters are connected in pairs, so that filtered water can be used for filling the underdrains and sand of the filters from below prior to starting, thus avoiding the disturbance which results from bringing dirty water upon the sand of a filter not filled with water. =Laboratory Building.=—The scientific control of filters is regarded as one of the essentials to the best results, and to provide for this there is a laboratory building at one end of the central court between the filters and close to the sedimentation-basin, supplied with the necessary equipment for full bacterial examinations, and also with facilities for observing the colors and turbidities of raw and filtered waters, and for making such chemical examinations as may be necessary. This building also provides a comfortable office, dark room, and storage room for tools, etc., used in the work.