The bottle shaped air vessels are used to produce an even, uniform discharge from the pump, the action of the pump plungers being intermittent. Air is stored inside the vessel, and the water, alter having passed through the delivery valve when the vessel is on the delivery pipe, compress ss the air. When the plunger make- the suction stroke, the air cushion acts as a spring and delivers water. A suction air vessel should be where the length of the suction pipe is great in comparison the diameter and for high-speed pumps, The contents of the air vessels vary in different make from thee times to ten times the capacity of the pump.