Today in 1880, US Patent 228292 A was issued, an invention of Otto Zwietusch, for his “Vent for Beer-Barrels.” There’s no Abstract, though it’s described this way in the application:

My invention relates to Vents for beer-barrels; and it consists in the device hereinafter described.

The drawing is a vertical section through the center of my device.

The bottle or bulb A may be of ordinary Construction, screw-threaded at its top to receive a rim, B, having an annular flange, the purpose of which is to clamp the cover O in place. A Washer is generally interposed between the bottle and cover, which latter is provided with an outlet-tube, D upon which l place the flexible pipe Gr, that leads into the beer-keg,and an outlet tube E, which extends down into bulb or bottle A, partly filled with water, and is provided with a flexible valve E,

which, while it will permit air to pass through from Without will be collapsed by greater pressure from within. So, therefore, when beer is being draw from the keg, the air rushes in from the outside to take its place; but as soon as the outflow from the keg ceases the valve E closes and prevents the exit of any of the gases which give life to the beer.