fit the two one end of an l shaped operating rod 7 is fixed to the inner end of the spindle 6 this connection of the ends is effected by welding or as indicated in the drawing by locking the end of the l shaped operating rod 7 in a groove made at the inner end of the spindle 6 in this arrangement by operating the operating lever 5 the l shaped operating rod 7 is turned with the spindle 6 as a pivot a sleeve 8 may be provided between the outer circumference of the spindle 6 and the inner surface of a hole in the side wall of the tank body 1 so that the spindle 6 may turn easily and smoothly by the operation of the operating lever in addition the other end of the operating lever 5 or the free end of the lever is equipped with a weight w so that that end of the lever 5 may always be perpendicular or vertical in consideration of the balance with the l shaped operating lever 7 this weight also acts as a handle when the operating lever 5 is operated the inner end of the l shaped operating rod 7 is coupled with one.