When the pressman pulls the handle II, he turns round the spindle M, and, by its connexion with the rod 0, Sce. the great lever turns with it, and causes the platen to descend and produce the requisite pressure. The power of the lever H is, however, transmitted to the screw, so as to be proportioned to the effect which is wanted at the dif ferent parts of the pull. At the beginning of the pull, for example, when motion only is wanted, the handle II lies in a direction parallel to the frame across the press, and the short lever N, which is nearly perpendicular to it, is also nearly perpendicular to the connecting rod 0 ; but the lever P of the screw makes a considerable angle with 0, and then it acts by a spindle radius to turn the screw. At the commencement, therefore, of the pull at H, the lever N acts with its full length upon a shorter length of lever on P, so that the screw will be turned more rapidly than if the link 0 were attached to it.' On continuing the pull, however, the situation of the levers changes, the length of P continually increasing in its acting length from its corn ing nearer to a perpendicular to 0, and the acting length of N diminishing, because, by the obliquity of the lever, the link 0 approaches the centre. The handle H like wise comes into a more favourable situation for the pull, as the pressman finally pulls in a direction nearly at right angles to its length. In this way the platen is at first brought quickly down upon the paper, where motion only is wanted; but as the levers are gradually coming into the most favourable position for exerting the greatest force, this maximum pressure is produced just at the moment when it is wanted, namely, when the platen touches the paper to be printed. The range of the handle is limited by a stop, which is moveable to a small extent, in order to vary the pressure for different kinds of work. The form of types, in place of resting upon a stone, lie on a cast iron block, which has its upper surface ground exactly flat, and placed perfectly horizontal.
In the Stanhope press the platen is so large as to print a whole sheet at one pull.
In sortie of these presses, a variation of power is obtain ed by a screw adjustment at the end of the link 0, by which it can he shortened. This is effected by fitting the centre pin, which unites it to the lever P in a bearing-piece, which slides in a groove formed on the side, and is regu lated by the screw. By this means, the descent of the platen may be increased or diminished. The surface of the platen is turned so as to be perfectly plane.
Since the invention of this press, various improvements have been made upon it, one of which, by Ai. de Heine, has been secured by a patent. In place of the screw, he has substituted a spiral or curved inclined plane, which is fixed to the head of the press. On the upper end of tire spindle is fixed a cross arm, which, acting against the fixed inclined plane, performs the functions of the screw. The advantage of this substitution is, that the acting bees admit of being made of hardened steel.
The Stanhope press has likewise received several valua ble improvements from Mr. Peter Keir. He forms the slider d, by boring out a cylindrical hole down the centre of the press, and he fits accurately into this a cylinder, to the lower end of which is fastened the platen. A flat side is made to the cylinder, which is prevented from turning round by a bar of iron screwed across the two cheeks, and bearing against the flat side of the cylinder. Mr. Keir
has also improved the lever apparatus. 13y a screw cut into the lower end of the spindle 1‘1, and fitted into a nut. the spindle is made to rise and fall through a space equal to the descent of the great screw, in the some time. The connecting rod 0 is thus made to pull in a horizontal plane, while in the old construction one end remains level when the other descends, the consequence of which is an unequal wearing of the joints.
An improvement on the printing press has also been made by Mr. Midhurst. It resembles generally the com mon printing press ; but the platen is the size of a sheet, and in place of a screw is used a plain spindle, on the lower part of which, just above the bar, is fixed a circular plate, which affords steps for the points of •two iron rods, which extend up to the head, and are supported in that place by their points entering sockets. These rods have an inclined position when the platen is raised, although both the ends of them are at the same distance from the centre of the spindle; but when the spindle is turned by the bar, the circular plate in which the lower points of the iron rods rest, moves round in a circle, and the upper ends remain stationary. Hence they conic towards a vertical position, and, by this motion, the spindle and platen are made to descend. The advantage of this contrivance is, that there is little friction, and that this power is hn mensely great when the rods come nearly parallel to the spindle.
Ruthven's Printing Press.
This very ingenious piece of mechanism, invented in 1813, by Mr. Ruthven, printer, in Edinburgh, possesses advantages which render it in many respects superior to any other. In all the presses which we have described, the motion of the carriage containing all the types and frame below the platen, must he considered as a piece of mechanism which should, if he avoided. ' To remove this necessity seems to have been the primary object of Mr. Ruthven. He seems to have resolved to keep the form of types fixed, and to make the platen moveable; and of course the whole mechanism of his press acquires its character from this fundamental im provement.
The construction of this press will be best understood from Plate. CCCCLXVIII. Figs. 4 and 5. where AA is the bed which sustains the form of types which remains fixed in the same position. This bed is supported by a framing of cast iron, as shown in the figure. The platen is shown at PP, and is joined to a strong cast iron bar, AIM by screws, s, s. At the ends of this bar there are strong iron bolts, b, b, secured by screw nuts at the top. These bolts terhiinate below in heads or projecting pieces, which lit exactly the hooks or clutches at E, E. At each of the remotest ends of the bar MM is fixed an oval steel spring, and on the lower ends of each spring are two grooved wheels or rollers, which run on a tail-way, so that the whole platen may he pushed off the form of types after it has given the pressure, and again brought above it to press the succeeding sheet. These springs have their elasticity and form so adjusted, that the lower surface of the platen is raised above the margin of the tympan, where the springs arc not in action. The platen, therefore, readily moves into its position immediately above the tympan, and as soon as it is pressed down upon the types, the action of the springs raises it above the margin of the tympan, and permits it to be pushed aside by the handle b from the form of types.