Air Vessel, in hydraulics, a contrivance to continue the flowing of water after the impelling force has ceased to act, as in the return stroke of a forcing pump, or in Bramah's hydrostatic press, thus preventing the shocks which would arise from the sudden stoppage of the water whilst in motion, and also avoiding the loss of power in moving it from a state of rest at each effective stroke. It consists of a vessel containing air, which is placed between the delivery valve and the mouth of the delivery pipe, and the water being forced through the pipe faster than it can escape at the orifice, rises in the air vessel, compressing the air therein with a pressure proportionate to the pressure on the delivery valve. On the return stroke, when the piston ceases to act, the air expands and continues the flow of water until the pressure of the air is only equal to the pressure of the column of water between the air vessel and the mouth of the delivery pipe. See the cut, in which a is the air vessel; b a flange by which it is attached to the delivery pipe or main; and c the exit pipe.