Packing in the stuffing-boxes, through which the piston-rods pass at the end of the cylinder, and so arranged to keep the cylinders steam-tight, at times become loose or displaced, and in such conditions steam may pass with considerable violence out of the cylinder by way of the stuffing-box, giving an acoustic effect somewhat resembling that of an engine with cylinder-cocks open, except that the noise heard is intermittent and not practically continuous, as when the cylinder cocks are open.