Dictionary.net

(Mech.) In the Suction-pump water is raised simply by the atmospheric pressure on the water in the well : on the up-stroke a vacuum is formed in the barrel or Suction-chamber, into which water is forced up by atmospheric pressure along the Suction-pipe ; on the down-stroke a valve at the top of the suctionpipe prevents the water from running back into the well ; it therefore forces its way through a valve in the piston into the space above, and at the next stroke is lifted to the spout.