term printing-preas is applied to the machine used for letter-press printing, lithographic printing, or copper-plate printing, but more usually the first, named, the others being called rolling-presses. The date of the invention of the printing-press is unknown, but some contrivance for this purpose must, have been used as soon as printing by blocks or types was introduced. The increased force requisite to make an impression, the size of the surface to be printed from being increased, would soon suggest recourse to some of the simple machines or mechanical powers for the modification of the power requisite to obtain the necessary pressure. The screw, as applied in the common screw-press, would obviously suggest itself ; and accordingly, in all the earlier printing-presses, the screw alone is used.
The operations to be performed in the process of printing will point out the essential parts of a printing-press. The types, being net up and arranged in a form of suitable dimensions, have to be inked ; this is effected by passing across them a cylinder, or roller, covered with au elastic composition of molasses, glue, and tar. The paper to be printed has to be laid on the types when inked, and then the requisite pressure for making the impression has to be applied.
The earliest form of printing-press very closely resembled the common screw-press, as the cheese or napkin press, with some contri vance for running the form of types, when inked, under the pressure, and back again when the impression was made. This rude and incon venient form of press was superseded by the invention of Blew, a printer of Amsterdam. Other improvements were from time to time introduced; but they were all superseded, about the commencement of the present century, by an invention of Lord Stanhope.
In the accompanying diagram of the Stanhope press, a is a massive frame of iron, cast in one piece, forming the body of the press, and firmly fixed to the cross n, or wooden foundation. The table c carries the form of types ro, which, being placed on s carriage, traverses the table backwards and forwards, motion being given to it by means of the crank-handle acting underneath the table. To the carriage are attached the tympana E, which are light frames covered with parch ment, and so constructed that the inner tympan just lies within the outer tympan. Some blanketing is placed between the tympana, so as to equalise the pressure upon the surface of the types. To the outer tympan is attached the frisket F. Tho sheet of paper to be printed being placed on the tympana, the frisket is turned down upon it ; and then tho frisket and tympans are turned down upon the form of types. The friskot is covered with paper or parchment, cut out so
that the sheet to be printed, when placed between the tympana and frisket, and folded down together on the form of types, may be in contact with the surface of the types ; while the remainder of tho friaket-shoet preserves the margin from being soiled.
The form of types being inked, and the tympana and frisket, with the sheet of paper between them, folded down on the form, the whole is run, by turning the crank-handle, under the platen o, which is a niaa-ive plate of cast-iron, moveable up and down perpendicularly, its weight being rather more than counterbalanced by the weight i at the back. The pressman pulls the handle of the barn towards him, or across the press, and thus communicates motion to k and 1, and causes the spindle In, which sustains the platen, to descend and produce the requisite pressure. The principal improvement of the Stanhope press consists in the manner in which the descending motion is given -to the screw. This depends on the properties of the bent lever, and may be explained in the following manner :—It is a necessary consequence of the peculiar combination and arrangement of the bent lever here employed, that on the handle u being moved, the platen descends rapidly at first; but as the platen comes Very near to the extreme point of its descent, the motion is extremely slow. But at this instant the platen is pressing the paper upon the types, and the pressure exerted being inversely as the rate of the descent of the platen, whose motion at this instant is exceedingly slow, the pressure produced is enormously large. It will be found also that at the Instant the platen is at its lowest point, the connecting bar 1, by which the power applied is transmitted to the platen, passes across the centres of motion of the system of forces ; at this instant, as theory points out, the ratio of the pressure produced to the power applied is indefinitely large. The pull having been made, or the pressure pro duced, the handle is returns to its original position, being taken back by the weight t at the back, which rather more than counterbalances the platen. The carriage is then run back, the frisket and tympana unfolded, and the printed sheet being taken out, the same operation is repeated. The usual rate of printing by the Stanhope press is two hundred and fifty per hour, two men being employed, one to ink the types, and the other as pressman.