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.
[Illustration:
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, 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.
[Illustration:
FIG. 162.—_Machine for Printing Paper-Hangings._
]
[Illustration:
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.
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.
[Illustration:
FIG. 164.—_Pascal’s Principle._
]
[Illustration:
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.
[Illustration:
FIG. 166.—_Hydraulic Press._
]
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.