l first formulated this remarkable
fact, of which a simple illustration is given in Fig. 179. Two
cylinders, A and B, having a bore of one and two inches respectively,
are connected by a pipe. Water is poured in, and pistons fitting the
cylinders accurately and of equal weight are inserted. On piston B is
placed a load of 10 lbs. To prevent A rising above the level of B, it
must be loaded proportionately. The area of piston A is four times that
of B, so that if we lay on it a 40-lb. weight, neither piston will move.
The walls of the cylinders and connecting pipe are also pressed outwards
in the ratio of 10 lbs. for every part of their interior surface which
has an area equal to that of piston B.

[Illustration: FIG. 179.]

[Illustration: FIG. 180.--The cylinder and ram of a hydraulic press.]

The hydraulic press is an application of this law. Cylinder B is
represented by a force pump of small bore, capable of delivering water
at very high pressures (up to 10 tons per square inch). In the place of
A we have a stout cylinder with a solid plunger, P (Fig. 180), carrying
the _table_ on which the object to be pressed is placed. Bramah, the
inventor of the hydraulic press, experienced great difficulty in
preventing the escape of water between the top of the cylinder and the
plunger. If a "gland" packing of the type found in steam-cylinders were
used, it failed to hold back the water unless it were screwed down so
tightly as to jam the plunger. He tried all kinds of expedients without
success; and his invention, excellent though it was in principle, seemed
doomed to failure, when his foreman, Henry Maudslay,[35] solved the
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.