The engraving illustrates an English invention of value in that it provides a means of giving vent to casks from which liquids are to be drawn, at the same time excluding the air when the drawing is discontinued, and thus preventing deterioration in the liquid by undue exposure to air.

The principle on which it operates is that of admitting just so much air as may be required to fill the vacant space produced by the withdrawal of the liquor from time to time, and affording this air no egress, thus hermetically sealing the barrel. This is effected by means of a valve opening inward, at the upper portion of the peg, so long as the density of the exterior air is in excess of that within. This action takes place at the very instant of the flow of the liquid, and ceases with it; for at that instant all further supply is shut off, there being no further pressure.