Discoveries and Inventions of the Nineteenth Century

works easily, without noise or jar, by turning the handle always in the same direction, producing 800 impressions in an hour. The bed moves backwards and forwards on wheels running on rails, the reciprocating movement being 320derived from the circular one by means of a crank. From the mode in which the table is carried backwards and forwards, the manufacturers call this the “Railway Printing Machine.” The paper is fed to the underside of the cylinder, which, after an impression has been given, remains stationary while the bed is returning, and while the layer-on is adjusting his sheet of paper. The axle of the impression cylinder carries a toothed wheel working in a rack on the bed or table, the wheel having at two parts of its circumference the teeth planed off so as to permit of the return of the table without moving the impression cylinder, which is again thrown into gear with the rack by a catch, so that the same tooth of the rack always enters the same space on the toothed wheel, and thus a good register is secured. The impression cylinder remains unaltered, whatever may be the size of the type form, it being only necessary to place the forward edge of the form always on the same line of the bed. Machines of a very similar construction, but driven by steam power, are used in lithographic printing; and in some of these machines advantage is elegantly taken of the fact that, when a wheel rolls along, the uppermost point of its circumference is always moving forward at exactly twice the velocity of its centre. Hence, if the table of a printing machine rests on the circumference of wheels, a backward and forward movement of the centres of these wheels, produced by the throw of a crank through a space of 2 ft., would produce a rectilineal reciprocating movement through a distance of 4 ft. of a table resting on the circumference of the wheels. Any reader who is interested in geometry or mechanics would do well to convince himself that the lowest point of the wheel of a railway carriage, for example, is stationary (considered while it is the lowest point), that the centre of the wheel is moving forwards with the velocity of the train, and that the highest point of the wheel is moving forwards with just twice the speed of the train. There is no difficulty about the rate of rectilineal motion of the centre, but the reader cannot possibly perceive the truth of the statement regarding the lowest and highest points unless he reflects on the subject, or puts it to the test of experiment. Another form of press which is used for good book printing is represented 321in the engraving, Fig. 160, which shows Napier’s platen machine. There the action is similar to that of the ordinary hand presses as regards the mode in which the paper is pressed against the face of the type; but the movements are all performed by steam power, applied through the driving belt, shown in the figure. The various kinds of printing machines adapted to each description of work are too numerous to admit of even a passing mention here; but those which have been described may fairly be considered as representing the leading principles of modern improvements. This article relates only to the mechanism by which an impression is transferred from a form to the surface of paper: the interesting and novel processes by which the form itself may be produced—processes which have amazingly abridged the printers’ labour and extended the resources of the art—deserve a separate chapter, and will furnish matter for an article on Printing Processes, which will be the better understood by being placed after chapters wherein the scientific bases of some of these processes are discussed. PATTERN PRINTING. The machines used for printing patterns are, in principle, very similar to those for letterpress printing; but the circumstance of several different colours having frequently to go to the production of one pattern leads to the multiplication, in the present class of machines, of the apparatus for distributing the colours and impressing the materials. Pattern printing machines are most extensively used for impressing fabrics, such as calicoes, muslins, &c., and for producing the wall-papers for decorating apartments. The machines employed for calicoes and for papers are so much alike, that to describe the one is almost to describe the other. The papers intended for paper-hangings are, in the first instance, covered with a uniform layer of the colour which is to form the ground, and this is done even in the case of papers which are to have a white ground. The colours thus laid on, and those which are applied by the machine, are composed of finely-ground colouring matters mixed with thin size or glue to a suitable consistence, and the ground-tint is given by bringing the upper surface of the paper, as it is mechanically unwound from a great roll, into contact with an endless band of cloth emerging from a trough containing a supply of the fluid colour. The paper then passes over a horizontal table, where the layer of colour is uniformly distributed over its surface by brushes moved by machinery, and the paper, after having been thoroughly dried, is ready to receive the impressions. The impressions may be given by flat blocks of wood on which the pattern is carved in relief, or from revolving cylinders on which the pattern is similarly carved. The former is the process of hand labour called “block printing,” and it requires much skill and care on the part of the operator; but with these, excellent results are obtained, as a correct adjustment of the positions of the parts of the pattern can always be secured. The latter is the mode of printing mechanically on rollers, corresponding with the type-bearing cylinders of the 322machines already described; but for pattern printing on paper they are made of fine-grained wood, mounted on an iron axle, and they are carved so that the design to be printed stands out in relief on their surface. One of these rollers is represented in Fig. 161, and it should be clearly understood that each colour in the pattern on a wall-paper requires a separate roller, the design cut on which corresponds only with the forms the particular colour contributes to the pattern. Such rollers being necessarily somewhat expensive, as the pattern is usually repeated many times over the cylindrical surface, the plan has been adopted of fastening a mass of hard composition in an iron axle, and when this has been turned to a truly cylindrical surface, it is made to receive plates of metal, formed of a fusible alloy of lead, tin, and nickel. These plates are simply casts from a single carved wooden mould of the pattern, which has thus only once to be formed by hand. The plates are readily bent when warmed, and are thus applied to the cylindrical surface, to which they are then securely attached. It is found advantageous to cover the prominent parts of the rollers which produce the impressions with a thin layer of felt, as this substance takes up the colours much more readily than wood or metal, and leaves a cleaner impression. Fig. 161.—Roller for Printing Wall-Papers. The machine by which wall-papers are printed is represented in Fig. 162, where it will be observed that the impression cylinder has a very large diameter, and that a portion of its circumference forms a toothed wheel, which engages a number of equal-sized pinions placed at intervals about its periphery. Each pinion being fixed on the axle of a pattern-bearing roller, these are all made to revolve at the same rate. There is, however, some adjustment necessary before that exact correspondence of the impressions with each other is secured, which is shown on the printed pattern by each colour being precisely in its appointed place. The rollers are constantly supplied with colour by endless cloths, which receive it from the troughs that are shown in the figure, one trough being appropriated to each roller. Some of these machines can print as many as eighteen or twenty different colours at once, by having that number of rollers; and it is easy to see how, by dividing each trough into several vertical compartments, in each of which a different colour is placed, it would be possible to triple or even quadruple the number of colours printed by one machine. The machinery by which calicoes are printed is almost identical in construction with that just described, and presents the same general appearance. There is, however, an important difference in the rollers, which in calico printing are of copper or bronze, and have the design engraved upon their polished cylindrical surface, not in relief, but in hollows. After the whole surface of the roller becomes charged with colour, there is in the machine a straight-edge, which removes the colour from the smooth surface, 323leaving only what has entered into the hollow spaces of the design, which, as the roller comes round to the cloth, yield it up to the surface of the latter. Thus, by a self-acting arrangement, the rollers are charged with colour, cleaned, and made to give up their impressions to the stuff by parting with the colour in the hollows. Rollers having patterns in relief are also used in calico printing, the mechanism being then almost identical with that of the former machine. It need hardly be said that great pains are taken in the construction of such machines to have each part very accurately adjusted, so that the impression may fall precisely upon the proper place, without any blurring or confusion of the colours, and the fact that an intricate design, having perhaps eighteen or twenty tints, can be thus mechanically reproduced millions of times speaks volumes for the accuracy and finish of the workmanship which are bestowed on such printing machines. Fig. 162.—Machine for Printing Paper-Hangings. 324 Fig. 163.—Chain-Testing Machine at Messrs. Brown and Lenox’s Works, Millwall. HYDRAULIC POWER. If a hollow sphere, a, Fig. 173, be pierced with a number of small holes at various points, and a cylinder, b, provided with a piston, c, fitted into it, when the apparatus is filled with water, and the piston is pushed inwards, the water will spout out of all the orifices equally, and not exclusively from that which is opposite to the piston and in the direction of its pressure. The jets of water so produced would not, as a matter of fact, all pursue straight paths radiating from the centre of the sphere, because gravity would act upon them; and all, except those which issued vertically, would take curved forms. But when proper allowance is made for this circumstance, each jet is seen to be projected with equal force in the direction of a radius of the sphere. This experiment proves that when pressure is applied to any part of a liquid, that pressure is transmitted in all directions equally. Thus the pressure of the piston—which, in the apparatus represented in the figure, is applied in the direction of the axis of the cylinder only—is carried throughout the whole mass of the liquid, and shows itself by its effect in urging the water out of the orifices in the sphere in all directions; and since the force with which the water rushes out is the same at every jet, it is plain that the water must press equally against each unit of area of the inside surface of the hollow sphere, without regard to the position of the unit. 325If we suppose the piston to have an area of one square inch, and to be pushed inwards with a force of 10 lbs., it cannot be doubted that the square inch of the inner surface of sphere immediately opposite the cylinder will receive also the pressure of 10 lbs.; and since the pressures throughout the interior of the hollow globe are equal, every square inch of its area