rotating the stem 58 so that ribs 68 see also fig8 and 11 formed longitudinally on the stem are misaligned with slots 63 in the passage 54 and engage the top of collar 56 to release valve 60 to the bias imposed by spring 62 the valve is rotated 90 degrees back to the position shown in fig8 so as to align the ribs 68 with the slots 63 and the valve will close other expedients as well may be used to hold the valve open such as detents clamps spring loaded pins etc when the tank assembly is placed on the base the valve 60 is automatically opened so as to allow water to flow from the tank into the receptacle 18 in the base 10 until the level of the water in the receptacle rises above lower surface 70 of collar 56 which will prevent additional air from flowing into the tank assembly through the passage 54 the interior of the tank assembly is sealed as there are no openings in the dome surface that allow air to enter the space above the water level in the dome however when the level of water in the receptacle drops below the surface 70 water in.