to be fed in stands on a high board at one end. The sheet is laid to its mark and is conveyed round an entry drum; thence it is carried round the first impression cylinder, and under this, moving at the same speed as the cylinder, is the type bed containing the inner forme already inked. The paper then receives its impression on the first side. In the older type of machine it is next led up to the right-hand one of the two reversing drums, which are placed above the large printing cylinders, and over which it passes with the printed side downwards. It is then brought under the second or left-hand drum, and so on to the other large impression cylinder, with the blank side of the sheet exposed to the type of the outer forme on the table underneath. Thus it will be seen that the sheet is reversed in its travel between the first and second large cylinders which give the impression. The sheet is then finally run out and delivered in the space between the two large cylinders, and laid on the delivery board—usually with the aid of flyers. In the more recent type of perfecting machines the sheet is fed directly into grippers, change taking place when grippers on each cylinder meet, the outer forme grippers taking the sheet from the inner forme grippers.
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Fig. 6.—Dryden & Foord's Perfecting (two-cylinders) Machine.
This is a general description of the principles on which these machines are built, but, as in other classes, there are many variations in details. For example, there are the drop-bar, the web and the gripper methods of feeding these presses. In the first case a bar descends upon the paper after it is laid to point marks, and this bar, having a rotary motion, runs the sheet between a roller and a small drum into the machine. The web arrangement consists of a series of broad tapes which lie on the laying-on board and are fastened to a small drum underneath it. This drum has a series of small cogs which move the web or tapes in the same direction. The sheet is laid to a back mark on the tapes, and is propelled between two rollers direct into the machine. Another variety employs grippers somewhat after the manner of the ordinary single cylinder. The Anglo-French perfecting machine is one of that class. As a rule most double-cylinder presses produce on an average about 1000 copies per hour, printed both sides.
Fig. 7.—The Miehle Two-revolution Cylinder Machine.
The two-revolution machine is another one-cylinder machine built on the reciprocating principle. Its speed is greater than the Two-Revolution Machines. stop cylinder (it may be geared to produce from 1500 to 2000 copies per hour, printed one side only). The Miehle (fig. 7), which is of American design but now made also in Great Britain, is a good example of this kind of machine and is much used, especially for illustrated work. It has the high over-feed board, and the taking-off apparatus is automatic but on a different plan from that of the ordinary Wharfedale, the sheets being carried over tapes with the freshly-printed side uppermost, thus preventing smearing; they are then carried on to the heap or pile by the frame or long arms placed at the end of the machine. A recent feature of this machine is the tandem equipment, whereby two, three or even four machines may be coupled together for colour work. Only one layer-on is required and register is obtained automatically throughout.
The principle of the two-revolution press is that the cylinder
