627,652. Gravity-separation apparatus. FARRER. Ltd.. W. E., and ROOKE, G. M. May 27, 1947, No. 14040. [Class 111] In a sewage settling tank of the kind having a small central sludge well and a stirrer in the well, the stirrer is carried at the lower end of a rotary spindle carried by, and mounted parallel with the rotation axis of, the scraper arm structure of the tank. In a circular settling tank having a sloping bottom 1, a central well 10 with a sloping wall 11 and outlet pipe 10a the stirrer, comprises one or more V- shaped arms 12 carried at the lower end of a spindle 13 driven through bevel gearing 16 by the spindle of one of two rollers 6 which support the inner end of a radial scraper structure 7 for rotation about a central diffuser box 5. The stirrer thus rotates both about its own axis and about a central pillar 2.