problem in a simple but most masterly manner. He had a recess turned in the neck of the cylinder at the point formerly occupied by the stuffing-box, and into this a leather collar of U-section (marked solid black in Fig. 180) was placed with its open side downwards. When water reached it, it forced the edges apart, one against the plunger, the other against the walls of the recess, with a degree of tightness proportionate to the pressure. On water being released from the cylinder the collar collapsed, allowing the plunger to sink without friction.
The principle of the hydraulic press is employed in lifts; in machines for bending, drilling, and riveting steel plates, or forcing wheels on or off their axles; for advancing the "boring shield" of a tunnel; and for other purposes too numerous to mention.
HOUSEHOLD WATER-SUPPLY FITTINGS.
Among these, the most used is the tap, or cock. When a house is served by the town or district water supply, the fitting of proper taps on all pipes connected with the supply is stipulated for by the water-works authorities. The old-fashioned "plug" tap is unsuitable for controlling high-pressure water on account of the suddenness with which it checks the flow. Lest the reader should have doubts as to the nature of a plug tap, we may add that it has a tapering cone of metal working in a tapering socket. On the cone being turned till a hole through it is brought into line with the channel of the tap, water passes. A quarter turn closes the tap.
Its place has been taken by the screw-down cock. A very common and effective pattern is shown in Fig. 181. The valve V, with a facing of rubber, leather, or some other sufficiently elastic substance, is