Twin-barrel air pumps became common in laboratories from the mid-eighteenth century onward and remained in use until the early twentieth century. Such pumps could not produce very strong vacuums. Despite many design variants, these devices were all based on the same principle.

The brass or glass barrels are fixed to a metal base connected, via a stop-cock and tubes, to the plate of the pump carrying the bell-jar to be evacuated. The barrels, fitted with leather collars, are operated by a handle carrying a pinion that meshes with the racks attached to the barrels.

When the right-hand piston is raised, the valve sealing the inlet to the plate opens, and the air is sucked out. The valve of the other piston stays shut. Reversing the movement, the valve of the right-hand piston closes, while the air contained in the piston is compressed and released into the atmosphere through a small valve fitted onto the piston. The left-hand piston functions in an identical manner, albeit in alternation with the right-hand piston: when one is in suction mode, the other is in compression mode, and vice versa.

By comparison with single-barrel pumps, the twin-barrel model provided faster extraction of air from the container to be evacuated. This type of machine made it possible to go from normal pressure, corresponding to a mercury column of about 760 millimeters, to a vacuum equivalent to a few millimeters of mercury.