to an easy stop and returns it without the
least vibration.

On all two-revolution presses there are employed, to assist in the
reversal of the bed, air-chambers or cylinders, without which the
reversing mechanisms could not withstand the enormous strain to which
they are subjected. These are iron cylinders, closed at one end,
approximately six inches in diameter and eighteen inches long, and
varying in size according to the size of the press. Some presses have
two and others four of these cylinders, one or two at each end. The
open ends of the cylinders are toward the bed, and attached to the bed
are two or four pistons which enter the air-chambers as the bed nears
the end of its stroke. The compression of the air in the cylinders
makes a cushion and checks the momentum of the moving bed. The pistons
can be adjusted to regulate the air compression to suit the velocity
of the bed and the weight of the form, which vary in different kinds
of work.

The delivery of the printed sheets is performed either by a delivery
cylinder or by a front delivery with the printed side of the paper
uppermost as already described for the stop-cylinder presses. Grippers
are not used in the front delivery carriage, as the sheet is
discharged from the cylinder by its continuous rotation.

The average running speed of a two-revolution press is about one-third
greater than that of a stop cylinder, or about eighteen hundred
impressions an hour, as against from one thousand to thirteen hundred
and fifty impressions from the stop cylinder, this being the
comparison in presses of the average size, printing sheets about
33 x 46 inches. The driving power required is in the proportion of
about five for the two-revolution press to three for the stop
cylinder, and the wear and tear is in about the same proportion.

Another press, which is still employed to a small extent for
book-work, is the flat-bed perfecting press. This press is virtually
two two-revolution presses combined into one, with the advantage that
they require only one man as "feeder," but with the disadvantage that
they produce only about two-thirds as much work as two separate
single-cylinder, two-revolution presses. Their greatest disadvantage
lies in the difficulty of preventing the fresh ink on the side of the
sheet first printed from "setting off" on the packing of the cylinder
which prints the reverse or second side. Mechanisms are employed to
move the "tympan sheet" or outsid