The air-pump is an instrument for removing air from a closed vessel. The closed vessel is called a receiver. It fits accurately upon a horizontal plate, through the centre of which is an opening communicating, by means of a bent tube, with a cylinder. An accurately fitting piston moves in this cylinder. At the junction of the bent tube with the cylinder, and in the piston, are two valves, opening from the receiver but not toward it. The tension of the air in the reciever, and the pressure of the air upon the valves, are equal. When the piston is raised, closes and the atmospheric pressure is removed from the valve. The tension of the air in the reciever opens valve one. By virtue of its power of indefinite expansion, the air which, at first, was in reciever and tube, now fills the reciever, tube, and cylinder. When the piston is pushed down, valve one closes, valve two opens, and the air in the cylinder escapes from the apparatus. (a.) The lower valve is sometimes supported, by a metal rod which passes through the piston. This rod works tightly in the piston, and is thus raised when the piston is raised, and lowered when the piston is lowered. A button near the upper end of this rod confines its motion within very narrow limits, allows valve one to be raised only a little, and compels the piston, during most of the journeys to and fro, to slide upon the rod instead of carrying the rod with it.