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 ma chines or mechanical powers for the modifi cation 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 set up and arranged in a chase (or iron frame) of suitable dimensions, which is then called a form, have to be inked ; this is effected by passing across them a cylinder, or roller, covered with an elastic composition of molasses and glue, which has been first rolled in a thick ink made of lamp .black and burnt oil. The paper to be printed has to be laid on the type when inked, and then the requisite pres sure for making the impression has to be applied.
Screw Press. The earliest form of printing press very closely resembled the common screw-press, as the cheese or napkin-press, with some contrivance for running the form of type, when inked, under the pressure, and back again when the impression was made. This rude and inconvenient form of press was superseded by the invention of Blew, a printer of Amsterdam. Other improvements were from time to time introduced ; but they were all superseded, about the commencement of the present century, by an invention of Lord Stanhope's.
lu the Stanhope Press, as in the common wooden screw-press, the form of type is laid upon a table which has a horizontal traversing motion. To the carriage on which the form moves are attached the tympans, which arc light frames covered with parchment, so con structed that the inner tympan just lies within the outer tympan.. Some blanketing is placed between the tympans, so as to equalise the pressure upon the surface of the types. To the outer tympan is attached a skeleton frame called the frisket. The sheet of paper to be printed being placed on the outer the frisket is turned down upon it ; and then the frisket and tympans are turned down upon the form of type. The frisket is covered with
paper, cut out so that the sheet to be printed, when placed between the tympan and frisket, and folded down together on the form, may be in contact with the surface of the type, while the remainder of the frisket-sheet pre serves the margin from being soiled. The form of type being inked, and the tympana and frisket, with the sheet of paper between them, folded down on the form, the whole is made to traverse, by moans of a crank-handle, until it comes beneath the platten, which is a massive plate of cast-iron, moveable up and down perpendicularly. The principal improve ment of the Stanhope Press consists in the manner in which the descending motion is given to the screw. The carriage is then run back, the frisket and tympans unfolded, and the printed sheet being taken out, the same operation is repeated.
The principle of the Stanhope press has been followed out by several subsequent in ventors, and improvements of mechanical detail introduced, tending to the economy of time and labour, and to precision of workman ship- In the Buthven Press, the form of type remains stationary, and the platten is removed to permit the type to be inked : and in this, as well as the Columbian Press, the pressure' is produced by a combination of levers alone,' without the use of any portion of a screw or inclined plane.
Copper Plate Press. The press fo'r copper plate printing consists of two cylinders or rollers of wood, supported in a strong wooden frame, and moveable about their axes, one placed just above and another just below the level of the table upon which the plate to be printed is laid. The upper roller is turned round by the arms of a cross fixed to its axis. The copper-plate being inked, the paper on which the impression is to be taken, and two or three folds of soft material, as blanketing, are placed upon it, The plate so prepared is moved along the table to the junction of the two rollers, and the upper roller being turned by the arms of the cross, the plate, with its furniture, is passed through the press. The rollers may be placed nearer to or farther from each other, according to the amount of pressure requisite for malting a good impres sion, that is, according to the depth of the engraving and the degree of blackness which the impression is required to have.