Chapter XIX: Introduction (17)

To such machines as those already described the world is indebted for cheap books, cheap newspapers, and cheap literature in general. But when, with railways and telegraphs, came the desire for the very latest intelligence, the necessities of the newspaper press, as regards rapidity of printing, soon required a greater speed than could possibly be attained by any of the flat form presses; for in these the table, with the forms placed upon it, is unavoidably of a considerable weight, and this heavy mass has to be set in motion, stopped, moved in the opposite direction, and again stopped during the printing of each sheet. The shocks and strains which the machine receives in these alternate reversals of the direction of the movement impose a limit beyond which the speed cannot be advantageously increased. When Mr. Applegath was again applied to by the proprietors of the “Times” to produce a machine capable of working off a still larger number of impressions, he decided upon abandoning the plan of reciprocating movement, and substituting a continuous rotary movement of the type form. And he successfully overcame the difficulties of attaching ordinary type to a cylindrical surface. The idea of placing the type on a rotating cylinder is due to Nicholson, who long ago proposed to give the types a wedge shape, so that the pieces of metal would, like the stones of an arch, exactly fit round the cylindrical surface. The wedge-shaped types were, however, so liable to be thrown from their places by the centrifugal force, that Nicholson proposed also certain mechanical methods of locking the types together after they had been placed on the circumference of the drum. The plan he suggested for this purpose involved, however, such an expenditure of time and trouble that his idea was never carried into practice. Mr. Applegath used type of the ordinary kind, which was set up on flat surfaces, forming the sides of a prism corresponding to the circumference of his revolving type cylinder, which was very large and placed vertically. The flat surfaces which received the type were the width of the columns of the newspaper, and the type forms were firmly locked up by screwing down wedge-shaped rules between the columns at the angles of the polygon. These form the “column rules,” which make the upright lines between the columns of the page, and by their shape they served to securely fix the type in its place. The diameter of the cylinder to which the form was thus attached was 5 ft. 6 in., but the type occupied only a portion of its circumference, the remainder serving as an inking table. Round the great cylinder eight impression rollers were placed, and to each impression roller was a set of inking rollers. At each turn, therefore, of the great cylinder eight sheets received the impression. These cylinders were, as already stated, placed vertically, and, as it was necessary to supply the sheets from horizontal tables, an ingenious arrangement of tapes and rollers was contrived, by which each sheet was first carried down from the table into a vertical position, with its plane directed towards the impression roller, in which position it was stopped for an instant, then moved horizontally forwards round the impression cylinder, and was finally brought out, suspended vertically, ready for a taker-off to place on his pile. This machine gave excellent results as to speed and regularity. From 10,000 to 12,000 impressions could be worked off in an hour, and the advantage was claimed for it of keeping the type much cleaner, by reason of its vertical position. The power of this machine may be judged of from one actual instance. It is stated that of copies of the “Times” in which the death of the Duke of Wellington was announced, 14th November, 1852, no less than 70,000 were printed in one day, and the machines were not once stopped, either to wash the rollers or to brush the forms. It may be mentioned, in order to give a better idea of the magnitude of the operation of printing this one newspaper, that one average day’s copies weigh about ten tons, and that the paper for the week’s consumption fills a train of twenty waggons.

At the “Times” office and elsewhere, the vertical machine has some years ago been superseded by others with horizontal cylinders. The fastest, perhaps, of all these printing machines is that which is now known as the “Walter Press,” so called either because its principle was suggested by the proprietor of the “Times,” or merely out of compliment to him. The improvements which are embodied in the Walter Press have been the subject of several patents taken out in the names of Messrs. MacDonald and Calverley, and it is to these improvements that we must now direct the attention of the reader. But we must premise that such machines as the Walter Press became possible only by the discovery of the means of rapidly producing what is called a stereotype plate from a form of type. A full account of the methods of effecting this is reserved for a subsequent article, but here it may suffice to say, that when a thick layer of moist cardboard, or rather a number of sheets of thin unsized paper pasted together and still quite moist, is forced down upon the form by powerful pressure, a sharp even mould of the type is obtained, every projection in the latter producing a corresponding depression in the _papier maché_ mould. When the paper mould is dry, it may be used for forming a _cast_ by pouring over it some fusible metallic alloy, having the properties of becoming liquid at a temperature which will not injure the mould, of taking the impressions sharply, and of being sufficiently hard to bear printing from. One of the improvements in connection with the Walter Press is in the mode of forming cylindrical stereotype casts from the paper mould. For this purpose the mould is placed on the _internal_ surface of an iron semi-cylinder, with the face which has received the impression of the type inwards. The central part of the semi-cylinder is occupied by a cylindrical iron core, which is adjusted so as to leave a uniform space between its convex surface and the concave face of the mould. Into this space is poured the melted metal, and its pressure forces the mould closely against the concave cylindrical surface to which it is applied, so that the thickness becomes quite uniform. The iron core has a number of grooves cut round it, and these produce in the cast so many ribs, or projections, which encircle the inner surface, and serve both to strengthen the cast and afford a ready means of obtaining an exact adjustment. Not the complete cylinder, but only half its circumference, is cast at once, the axis of the casting apparatus being placed horizontally, and the liquid metal poured in one unbroken stream between the core and the mould from a vessel as long as the cylinders. Fig. 156 is a section of the casting apparatus, in which _a_ is the core, _b_ the _papier maché_ mould, _c_ the iron semi-cylinder containing it, _d_ the metal which has been poured in at the widened space, _e_. When the metal has solidified, the core is simply lifted off, and the cast is then taken out, in the form of a semi-cylinder, the internal surface of which has exactly the diameter of the external surface of the roller of the machine on which it is to be placed, in company with another semi-cylindrical plate, so that the two together encircle half the length of the roller, and when another pair of semi-cylinders have been fixed on the other part of the roller, the whole matter of one side of the newspaper sheet, usually four pages, is ready for printing. One great advantage of working from stereotype casts made in this way is that the form-bearing cylinder of the machine has no greater circumference than suffices to afford space for the matter on one side of the paper. The casts are securely fixed on the revolving cylinder by elbows, which can be firmly screwed down. The casts are usually made to contain one page each, so that four semi-cylinders, each half the length of the revolving cylinder, are fixed on the circumference of the latter. The process of casting in no way injures the paper mould, which is in fact generally employed to produce several plates.

FIG. 156.
]

FIG. 157.—_Diagram of the Walter Press._
]

The Walter Machine is not fed with separate sheets of paper, but takes its supply from a huge roll, and itself cuts the paper into sheets after it has impressed it on both sides. This is done by a very simple but effective plan, which consists in passing the paper between two equal-sized rollers, the circumference of which is precisely the length of the sheets to be cut. These rollers grip the paper, but only on the marginal spaces; and on the circumference of one of them, and parallel to its axis, is a slightly projecting steel blade, which fits into a corresponding recess, or groove, in the circumference of the other, and at this time the whole width of the sheet is firmly held by a projecting piece acted on by a spring. Although the Walter Machine, as actually constructed, presents to the uninitiated spectator an apparently endless and intricate series of parallel cylinders and rollers, yet it is in reality exceedingly simple in principle, as may be seen by the diagram given in Fig. 157. In this we may first direct the reader’s attention to the two cylinders, F_{1}, F_{2}, which bear the stereotype casts—one of the matter belonging to one side of the sheet, the other of the matter belonging to the other side, for the Walter Press is a perfecting machine—and the web of paper having been printed by F_{1}, against which it is pressed by the roller, P_{1}, passes straight, as shown by the dotted line, to the second pair of cylinders, in order to be printed on the other side; and here, of course, the form cylinder, F_{2}, is below, and the impression cylinder, P_{2}, above, and an endless cleaning blanket is supplied to the latter to receive the _set-off_. The web of paper then passes between the cutting rollers, C, C_{1}, by which it is cut in sheets. But the knife has a narrow notch in the centre, and one at each end, so that the paper is not severed at those parts, narrow strips or tags being left, which maintain for a while a slight connection. But the tapes, _t_{1}_, _t_{2}_, between which the paper is now carried, are driven at a rather quicker rate than the web issues from C, C_{1}; and the result is, that the tags are torn, and the sheet becomes separated from the portion next following it. Thus, as a separate sheet, it arrives at the horizontal tapes, _h_, and is brought to another set of tapes mounted on the frame, _r_, rocking about the centre, _c_, by which it is brought finally to the tapes, _f_{1}_, _f_{2}_, which by the movement of _r_ receive the sheets alternately. A sheet-flyer, _s_, oscillates between the tapes, _f_{1}_, _f_{2}_; and as fast as the sheets arrive, lays them down right and left alternately, and it only remains for the piles, _p_{1}_, _p_{2}_ so formed, to be removed. The inking apparatus of each form-cylinder is indicated by the series of rollers marked I_{1}, I_{2}; and in this part of the machine there are also some improvements over former presses, for the distributing rollers are not made of composition, but of iron, turned with great exactness to a true surface, and arranged so as not quite to touch each other. At D is an apparatus for damping the paper, in which there are hollow perforated cylinders, covered by blanket, and filled with some porous material, which is kept constantly wet. These cylinders being made to rotate rapidly, the centrifugal force causes the water to find its way uniformly to the outside. Here the paper also passes between rollers intended to flatten and to stretch it. At R is the great roll of paper, from which the machine takes its supply. These rolls contain, perhaps, five miles length of paper, and at first it was a matter of some difficulty to fix them firmly on their wooden axles, so that they might be steadily unwound; but the contrivers of the Walter Press make these spindles as tight as may be required by forming them in wedge-shaped pieces, which can be made to increase the thickness of the spindle by drawing one upon another by screws.

The great speed of the Walter Machine is secured by the paper being drawn by the machine itself from a continuous web, instead of being laid on in a separate sheet, so that the machine is not dependent on the dexterity of the layers-on, who are besides necessarily highly-skilled workmen, and therefore a great economy of wages results from using a machine which does not require their services; and as the Walter Press also itself lays down the perfected sheets, the necessary attendants are as few as possible. The waste of paper and loss of time by stoppages are said to be extremely small with this machine.

Fig. 148 will give some idea of the appearance of the printing-room where one of the leading London daily papers is being printed by Walter Presses.

Another fast printing machine is the type revolving cylinder machine invented by Colonel Richard M. Hoe, and manufactured by the well-known firm of Hoe and Company, New York, with whose name the history of fast printing machines must ever be associated. In these machines the type is placed on the circumference of a cylinder which rotates about a horizontal axis, and the difficulties of securely locking up the type are successfully overcome. The machines are made with two, four, six, eight, or ten impression cylinders, and at each revolution of the great cylinder the corresponding number of impressions are produced. The engraving on the opposite page, Fig. 158, represents the two-cylinder machine, and an examination of the figure will render its general action intelligible. The form of type occupies about one-fourth of the circumference of the great cylinder, the remainder being used as an ink-distributing surface. Round this main cylinder, and parallel to it, are placed smaller impression cylinders, from two to ten in number, according to the size of the machine. When the press is in operation, the rotation of the main cylinder carries the type form to each impression cylinder in succession, and it there impresses the paper, which is made to arrive at the right time to secure true register. One person is required for each impression cylinder, to supply the sheets of paper, which have merely to be laid in a certain position, when, at the proper moment, they are seized by the “grippers,” or fingers of the machine, and after having been printed, are carried out by tapes, and laid in heaps by self-acting sheet-flyers, by which the hands which are required to receive and pile the sheets in other machines are dispensed with. The ink is contained in a fountain placed beneath the main cylinder, and is conveyed by means of rollers to the distributing surface of the main cylinder. This surface, being lower than that of the type forms, passes by the impression cylinders without touching them. For each impression cylinder there are two inking rollers, receiving their supply of ink from the distributing surface of the main cylinder. These inking rollers, the bearings of which are, by springs, drawn towards the axis of the main cylinder, rise as the form passes under them, and having inked it, they again drop on to the distributing surface. Each page of the matter is locked up on a detachable segment of the large cylinder, which segment constitutes its bed and chase. The column-rules are parallel with the shaft of the cylinder, and are consequently straight, while the head, advertising, and dark rules have the form of segments of a circle. The column-rules are in the shape of a wedge, with the thin end directed towards the axis of the cylinder, so as to bind the types securely. These wedge-shaped column-rules are held in their place by tongues projecting at intervals along their length, and sliding in grooves cut crosswise in the face of the bed. The spaces in the grooves between the column-rules are accurately fitted with sliding blocks of metal level with the surface of the bed, the ends of the blocks being cut away underneath, to receive a projection on the sides of the tongues of the column-rules. The locking up is effected by means of screws at the foot of each page, by which the type is held as securely as in the ordinary manner upon a flat bed. The main cylinder of the machine represented in Fig. 158 has a diameter of 3 ft. 9 in., and its length is, according to the size of the sheets to be printed, from 4 ft. 5 in. to 7 ft. 4 in. The whole is about 20 ft. long, 10 ft. wide, including the platforms, and a height of 9 ft. in the room in which it is placed suffices for its convenient working. The steam power required is from one to two horse-power, according to the length of the main cylinder. The speed of these machines is limited only by the ability of the feeders to supply the sheets fast enough. The ten-cylinder machine has, of course, ten impression cylinders, instead of two, and there are ten feeding-tables, arranged one above the other, five on each side. The main cylinder has a diameter of 4 ft. 9 in., and is 6 ft. 8 in. long. The machine occupies altogether a space of 31 ft. by 16 ft., and its height is 18 ft. A steam engine of eight horse-power is sufficient to drive the ten-cylinder machine, which is then capable of producing 25,000 impressions per hour. The mechanism of the larger machines is precisely similar to that of the two-cylinder machine, except such additional devices as are necessary to carry the paper to and from the main cylinder at four, six, eight, or ten points of its circumference. Much admirable contrivance is displayed in the manner of disposing feeders as closely as possible round the central cylinder.

FIG. 158.—_Messrs. Hoe’s Type Revolving Cylinder Machine._
]

In some machines, such as Messrs. Hoe’s, Fig. 158, the sheet-flyers are interesting features, for they form an efficient contrivance for laying down and piling up, with the greatest regularity, sheet after sheet as it issues from the press. The sheet-flyer is in fact an automatic taker-off, and therefore it supersedes the services of the boy who would otherwise be required. It is simply a light wooden framework of parallel bars, turning on one of its sides as a centre; and the tapes carrying the sheet, passing down between the bars, bring the paper down upon the frame, where its progress is then stopped, the frame makes a rapid turn on its centre, lays down the sheet, and quickly rises to receive another from the tapes. One can hardly see a printing machine in action without being struck with the deftness with which the sheet-flyer does its duty; for the precision with which it receives a sheet, lays it down, and then quickly returns, to be ready for the next, suggest to the mind of the spectator rather the movements of a conscious agent than the motions of an unintelligent piece of mechanism. The sheet-flyer is seen at the left-hand side of Fig. 158, where it is in the act of laying down a sheet on the pile it has already formed.

FIG. 159.—_Messrs. Hoe’s “Railway” Machine._
]

FIG. 160.—_Napier’s Platen Machine._
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The modern improvements in printing presses are well illustrated by the machine represented on the opposite page, Fig. 159, which has been designed by the Messrs. Hoe to work exclusively by hand. It is intended for the newspaper and job work of a country office, and it 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 derived 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 in 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 machines 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._
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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, leaving 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._
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FIG. 163.—_Chain-Testing Machine at Messrs. Brown and Lenox’s Works,
Millwall._
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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.

If 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 will also be pressed outwards with a force equal to 10 lbs. Hence, if the total area of the interior be 100 square inches, the whole pressure produced will amount to a hundred times 10 lbs.

FIG. 164.—_Pascal’s Principle._
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FIG. 165.—_Collar of Hydraulic Cylinder._
]

That water or any other liquid would behave in the manner just described might be deduced from a property of liquids which is sufficiently obvious, namely, the freedom with which their particles move or slide upon each other. The equal transmission of pressure in all directions through liquids was first clearly expressed by the celebrated Pascal, and it is therefore known as “Pascal’s principle.” He said that “if a closed vessel filled with water has two openings, one of which is a hundred times as large as the other; and if each opening be provided with an exactly-fitting piston, a man pushing in the small piston could balance the efforts of a hundred men pushing in the other, and he could overcome the force of ninety-nine.” Pascal’s principle—which is that of the hydraulic press—may be illustrated by Fig. 164, in which two tubes of unequal areas, _a_ and _b_, communicate with each, and are supposed to be filled with a liquid—water, for example, which will, of course, stand at the same level in both branches. Let us now imagine that pistons exactly fitting the tubes, and yet quite free to move, are placed upon the columns of liquid—the larger of which, _b_, we shall suppose to have five times the diameter, and therefore twenty-five times the sectional area, of the smaller one. A pressure of 1 lb. applied to the smaller piston would, in such a case, produce an upward pressure on the larger piston of 25 lbs.; and in order to keep the piston at rest, we should have to place a weight of 25 lbs. upon it. Here then a certain force appears to produce a much larger one, and the extent to which the latter may be increased is limited only by the means of increasing the area of the piston. Practically, however, we should not by any such arrangement be able to prove that there is exactly the same proportion between the total pressures as between the areas, for the pistons could not be made to fit with sufficient closeness without at the same time giving rise to so much friction as to render exact comparisons impossible. We may, however, still imagine a theoretical perfection in our apparatus, and see what further consequences may be deduced, remembering always that the actual results obtained in practice would differ from these only by reason of interfering causes, which can be taken into account when required. We have supposed hitherto that the pressures of the pistons exactly balance each other. Now, so long as the system thus remains in equilibrium no _work_ is done; but if the smallest additional weight were placed upon either piston, that one would descend and the other would be pushed up. As we have supposed the apparatus to act without friction, so we shall also neglect the effects due to difference in the levels of the columns of liquid when the pistons are moved; and further, in order to fix our ideas, let us imagine the smaller tube to have a section of 1 square inch in area, and the larger one of 25 square inches. Now, if the weight of the piston, _a_, be increased by the smallest fraction of a grain, it will descend. When it has descended a distance of 25 in., then 25 cubic inches of water must have passed into _b_, and, to make room for this quantity of liquid, the piston with the weight of 25 lbs. upon it must have risen accordingly. But since the area of the larger tube is 25 in., a rise of 1 in. will exactly suffice for this; so that a weight of 1 lb. descending through a space of 25 in., raises a weight of 25 lbs. through a space of 1 in. This is an illustration of a principle holding good in all machines, which is sometimes vaguely expressed by saying that _what is gained in power is lost in time_. In this case we have the piston, _b_, moving through the space of 1 in. in the same time that the piston a moves through 25 in.; and therefore the _velocity_ of the latter is twenty-five times greater than that of the former, but the time is the same. It would be more precise to say, that what is gained in force is lost in space; or, that no machine, whatever may be its nature or construction, is of itself capable of doing _work_. The “mechanical powers,” as they are called, can do but the work done upon them, and their use is only to change the relative amounts of the two factors, the product of which measures the work, namely, space and force. Pascal himself, in connection with the passage quoted above, clearly points out that in the new mechanical power suggested by him in the hydraulic press, “the same rule is met with as in the old ones—such as the lever, wheel and axle, screw, &c.—which is, that the distance is increased in proportion to the force; for it is evident that as one of the openings is a hundred times larger than the other, if the man who pushes the small piston drives it forward 1 in., he will drive backward the large piston one-hundredth part of that length only.” Though the hydraulic press was thus distinctly proposed as a machine by Pascal, a certain difficulty prevented the suggestion from becoming of any practical utility. It was found impossible, by any ordinary plan of packing, to make the piston fit without allowing the water to escape when the pressure became considerable. This difficulty was overcome by Bramah, who, about the end of last century, contrived a simple and elegant plan of packing the piston, and first made the hydraulic press an efficient and useful machine. Fig. 166 is a view of an ordinary hydraulic press, in which a is a very strong iron cylinder, represented in the figure with a part broken off, in order to show that inside of it is an iron piston or ram, _b_, which works up and down through a water-tight collar; and in this part is the invention by which Bramah overcame the difficulties that had previously been met with in making the hydraulic press of practical use. Bramah’s contrivance is shown by the section of the cylinder, Fig. 165, where the interior of the neck is seen to have a groove surrounding it, into which fits a ring of leather bent into a shape resembling an inverted U. The ring is cut out of a flat piece of stout leather, well oiled and bent into the required shape. The effect of the pressure of the water is to force the leather more tightly against the ram, and as the pressure becomes greater, the tighter is the fit of the collar, so that no water escapes even with very great pressures. To the ram, _b_, Fig. 166, a strong iron table, _c_, is attached, and on this are placed the articles to be compressed. Four wrought iron columns, _d d d d_, support another strong plate, _e_, and maintain it in a position to resist the upward pressure of the goods when the ram rises, and they are squeezed between the two tables. The interior of the large cylinder communicates by means of the pipe, _f f_, with the suction and force-pump, _g_, in which a small plunger, _o_, works water-tight. Suppose that the cylinders and tubes are quite filled with water, and that the ram and piston are in the positions represented in the figure. When the piston of the pump, _g_, is raised, the space below it is instantly filled with water, which enters from the reservoir, _h_, through the valve, _i_, the valve _k_ being closed by the pressure above it, so that no water can find its way back from the pipe, _f_, into the small cylinder. When the piston has completed its ascent, the interior of the small cylinder is therefore completely filled with water from the reservoir; and when the piston is pushed down, the valve, _i_, instantly closes, and all egress of the liquid in that direction being prevented, the greater pressure in _g_ forces open the valve, _k_, and the water flows along the tube, _f_, into the large cylinder. The pressure exerted by the plunger in the small cylinder, being transmitted according to the principles already explained, produces on each portion of the area of the large plunger equal to that of the smaller an exactly equal pressure. In the smaller hydraulic presses the plunger of the forcing-pump is worked by a lever, as represented in the figure at _n_; so that with a given amount of force applied by the hand to the end of the lever, the pressure exerted by the press will depend upon the proportion of the sectional area of _b_ to that of _o_, and also upon the proportion of the length _m n_, to the length _m l_. To fix our ideas, let us suppose that the distance from _m_ of the point _n_ where the hand is applied is ten times the distance _m l_, and that the sectional area of _b_ is a hundred times that of _o_. If a force of 60 lbs. be applied at _n_, this will produce a downward pressure at _m_ equal to 60 × 10, and then the pressure transmitted to the ram of the great cylinder will be 60 × 10 × 100 = 60,000 lbs. The apparatus is provided with a safety-valve at _p_, which is loaded with a weight; so that when the pressure exceeds a desired amount, the valve opens and the water escapes. There is also an arrangement at _q_ for allowing the water to flow out when it is desired to relieve the pressure, and the water is then forced out by the large plunger, which slowly descends to occupy its place. The body of the cylinder is placed beneath the floor in such presses as that represented in Fig. 166, in order to afford ready access to the table on which the articles to be compressed are placed.

FIG. 166.—_Hydraulic Press._
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The force which may, by a machine of this kind, be brought to bear upon substances submitted to its action, is limited only by the power of the materials of the press to resist the strains put upon them. If water be continually forced into the cylinder of such a machine, then, whatever may be the resistance offered to the ascent of the plunger, it must yield, or otherwise some part of the machine itself must yield, either by rupture of the hydraulic cylinder, or by the bursting of the connecting-pipe or the forcing-pump. This result is certain, for the water refuses to be compressed, at least to any noticeable degree, and therefore, by making the area of the plunger of the force-pump sufficiently small, there is no limit to the pressure per square inch which can be produced in the hydraulic cylinder; or, to speak more correctly, the limit is reached only when the pressure in the hydraulic cylinder is equal to the cohesive strength of the material (cast or wrought iron) of which it is formed. It has been found that when the internal pressure per square inch exceeds the cohesive or tensile strength of a rod of the metal 1 in. square (see page 207), no increase in the thickness of the metal will enable the cylinder to resist the pressure. Professor Rankine has given the following formula for calculating the external radius, R, of a hollow cylinder of which the internal radius is _r_, the pressure per square inch which it is desired should be applied before the cylinder would yield being indicated by _p_, while _f_ represents the tensile strength of the materials:

R = _r_√((_f_ + _p_)/(_f_ – _p_))

We may see in this formula that as the value of _p_ becomes more and more nearly equal to _f_, the less does the divisor (_f_ – _p_) become, and therefore the greater is the corresponding value of R; and when _f_ = _p_, or _f_ – _p_ = 0, the interpretation would be that no value of R would be sufficiently great to satisfy the equation. Thus a cylinder, made of cast iron, of which the breaking strain is 8 tons per square inch, would have its inner surface ruptured by that amount of internal pressure, and the water passing into the fissures would exert its pressure with ever-increasing destructive effect.

With certain modifications in the proportions and arrangement of its parts, the hydraulic press is used for squeezing the juices from vegetable substances, such as beetroots, &c., for pressing oils from seeds, and, in fact, all purposes where a powerful, steady, and easily regulated pressure is needed. Cannons and steam boilers are tested by hydraulic pressure, by forcing water into them by means of a force-pump, just as it is forced into the cylinder of the hydraulic press described above. This mode of testing the strength has several great advantages; for not only can the pressure be regulated and its amount accurately known; but in case the cannon or steam boiler should give way, there is no danger, for it does not explode—the metal is simply ruptured, and the moment this takes place, the water flows out and the strain at once ceases.

The strength of bars, chains, cables, and anchors is also tested by hydraulic power, and the engraving at the head of this article, Fig. 163, represents the hydraulic testing machine at the works of Messrs. Brown and Lenox, the eminent chain and anchor manufacturers, of Millwall. Immediately in front of the spectator are the force-pumps, and the steam engine by which they are driven. It will be observed that four plungers are attached to an oscillating beam in such a manner that the water is continuously forced into the hydraulic cylinder. The outer pair of plungers are of much larger diameter than the inner pair, in order that the supply of water may be cut off from the former when the pressure is approaching the desired limit, and the smaller pair alone then go on pumping in the water, the pressure being thus more gradually increased. Behind the engine and forcing pump is the massive iron cylinder, where the pressure is made to act on a piston, which is forced towards that end of the cylinder seen in the drawing. The piston is attached to a very thick piston-rod, moving through a water-tight collar at the other end of the cylinder. The effect of the hydraulic pressure is, therefore, to draw the piston-rod into the cylinder, and not, as in the apparatus represented in Fig. 166, to force a plunger out. The head of the piston-rod is provided with a strong shackle, to which the chains to be tested can be attached. In a line with the axis of the cylinder is a trough, some 90 ft. long, to hold the chain, and at the farther end of the trough is another very strong shackle, to which the other end of the chain is made fast. A peculiarity of Messrs. Brown and Lenox’s machine is the mode in which the tension is measured. In many cases it is deemed sufficient to ascertain by some kind of gauge the pressure of the water in the hydraulic cylinder, and from that to deduce the pull upon the chain; but the Messrs. Brown have found that every form of gauge is liable to give fallacious indications, from variations of temperature and other circumstances, and they prefer to measure the strain directly. This is accomplished by attaching the shackle at the farther extremity of the trough to the short arm of a lever, turning upon hard steel bearings, the long arm of this lever acting upon the short arm of another, and so on until the weight of 1 lb. at the end of the last lever will balance a pull on the chain of 2,240 lbs., or 1 ton. The tension is thus directly measured by a system of levers, exactly resembling those used in a common weighing machine, and this is done so accurately that even when a chain is being subjected to a strain of many tons, an additional pull, such as one can give to the shackle-link with one hand, at once shows itself in the weighing-room. The person who has charge of this part of the machine places on the end of the lever a weight of as many pounds as the number of tons strain to which the chain to be tested has to be submitted. The engineer sets the pump in action, the water is rapidly forced into the cylinder, the piston is thrust inwards, and the strain upon the chain begins; the engineer then cuts off the water supply from the larger force-pumps, and the smaller pair go on until the strain becomes sufficient to raise the weight, and then the person in the weighing-room, by pulling a wire, opens a valve in connection with the hydraulic cylinder, which allows the water to escape, and the strain is at once taken off. This testing machine, which is capable of testing cables up to 200 tons or more, was originally designed by Sir T. Brown, the late head of the firm, and not only was the first constructed in the country, but remains unsurpassed in the precision of its indications.

The testing of cables, which we have just described, is a matter of the highest importance, for the failure of cables and anchors places ships and men’s life in great danger, since vessels have frequently to ride out a storm at anchor, and should the cables give way, a ship would then be almost entirely at the mercy of the winds and waves. Hence the Government have, with regard to cables and anchors, very properly made certain stringent regulations, which apply not only to the navy but to merchant shipping. The chain-cable is itself a comparatively modern application of iron, for sixty years ago our line-of-battle ships carried only huge hempen cables of some 8 in. or 9 in. diameter. Chain-cables have now almost entirely superseded ropes, though some ships carry a hempen cable, for use under peculiar circumstances. The largest chain-cables have links in which the iron has a diameter of nearly 3 in., and these cables are considered good and sound when they can bear a strain of 136 tons. Such are the cables used in the British navy for the largest ships. Of course, there are many smaller-sized cables also in use, and the strains to which these are subjected when they are tested in the Government dockyards vary according to the thickness of the iron; but it is found that nearly one out of every four cables supplied to the Admiralty proves defective in some part, which has to be replaced by a sounder piece. The chain-cables made by Messrs. Brown and Lenox for the _Great Eastern_ are, as might have been expected, of the very stoutest construction; the best workmanship and the finest quality of iron having been employed in their manufacture. These cables were tested up to 148 tons, a greater strain than had ever before been applied as a test to any chain, and it was found that a pull represented by at least 172 tons was required to break them. It is difficult to believe that a teacup-full of cold water shoved down a narrow pipe is able to rend asunder the massive links which more than suffice to hold the huge ship securely to her anchors, but such is nevertheless the sober fact. The regulations of the Board of Trade require that every cable or anchor sold for use in merchant ships is to be previously tested by an authorized and licensed tester, who, if he finds it bears the proper strain, stamps upon it a certain mark.