It is often desirable to force air into or remove it from a vessel. Air is forced into a vessel by machines called air pumps, air compressors, condensing pumps, and blowing engines or blowers. The air pump consists of a tube or pipe with a rim, ground smooth and flat, extending from the cylinders.

Notice the tire of an automobile as air is pumped into it. As the air enters, the tube expands, due to the pressure of the gas, until finally the pressure becomes great enough to support the weight of the automobile. To remove air from a vessel, a screw connection is fitted tightly to it. As the piston is drawn up a partial vacuum is caused by the pressure of the air underneath, so that the air from the vessel immediately rushes to the cylinder, forcing the valve upward. This continues until the air pressure is reduced to such an extent that it is unable to force the valve of the cylinder open.