On the Economy of Machinery and Manufactures - Charles Babbage (interesting books to read in english .TXT) 📗

into any

required pattern. If the block be placed upon a piece of fine

woollen cloth, on which ink of any colour has been uniformly

spread, the projecting copper wires receive a portion, which they

give up when applied to the calico to be printed. By the former

method of printing on calico, only one colour could be used; but

by this plan, after the flower of a rose, for example, has been

printed with one set of blocks, the leaves may be printed of

another colour by a different set.

 

97. Printing oilcloth. After the canvas, which forms the

basis of oilcloth, has been covered with paint of one uniform

tint, the remainder of the processes which it passes through, are

a series of copyings by surface printing, from patterns formed

upon wooden blocks very similar to those employed by the calico

printer. Each colour requiring a distinct set of blocks, those

oilcloths with the greatest variety of colours are most

expensive.

 

There are several other varieties of printing which we shall

briefly notice as arts of copying; which, although not strictly

surface printing, yet are more allied to it than that from

copperplates.

 

98. Letter copying. In one of the modes of performing this

process, a sheet of very thin paper is damped, and placed upon

the writing to be copied. The two papers are then passed through

a rolling press, and a portion of the ink from one paper is

transferred to the other. The writing is, of course, reversed by

this process; but the paper to which it is transferred being

thin, the characters are seen through it on the other side, in

their proper position. Another common mode of copying letters is

by placing a sheet of paper covered on both sides with a

substance prepared from lamp-black, between a sheet of thin paper

and the paper on which the letter to be despatched is to be

written. If the upper or thin sheet be written upon with any hard

pointed substance, the word written with this style will be

impressed from the black paper upon both those adjoining it. The

translucency of the upper sheet, which is retained by the writer,

is in this instance necessary to render legible the writing which

is on the back of the paper. Both these arts are very limited in

their extent, the former affording two or three, the latter from

two to perhaps ten or fifteen copies at the same time.

 

99. Printing on china. This is an art of copying which is

carried to a very great extent. As the surfaces to which the

impression is to be conveyed are often curved, and sometimes even

fluted, the ink, or paint, is first transferred from the copper

to some flexible substance, such as paper, or an elastic compound

of glue and treacle. It is almost immediately conveyed from this

to the unbaked biscuit, to which it more readily adheres.

 

100. Lithographic printing. This is another mode of producing

copies in almost unlimited number. The original which supplies

the copies is a drawing made on a stone of a slightly porous

nature, the ink employed for tracing it is made of such greasy

materials that when water is poured over the stone it shall not

wet the lines of the drawing. When a roller covered with printing

ink, which is of an oily nature, is passed over the stone

previously wetted, the water prevents this ink from adhering to

the uncovered portions; whilst the ink used in the drawing is of

such a nature that the printing ink adheres to it. In this state,

if a sheet of paper be placed upon the stone, and then passed

under a press, the printing ink will be transferred to the paper,

leaving the ink used in the drawing still adhering to the stone.

 

101. There is one application of lithographic printing which

does not appear to have received sufficient attention, and

perhaps further experiments are necessary to bring it to

perfection. It is the reprinting of works which have just arrived

from other countries. A few years ago one of the Paris newspapers

was reprinted at Brussels as soon as it arrived by means of

lithography. Whilst the ink is yet fresh, this may easily be

accomplished: it is only necessary to place one copy of the

newspaper on a lithographic stone; and by means of great pressure

applied to it in a rolling press, a sufficient quantity of the

printing ink will be transferred to the stone. By similar means,

the other side of the newspaper may be copied on another stone,

and these stones will then furnish impressions in the usual way.

If printing from stone could be reduced to the same price per

thousand as that from moveable types, this process might be

adopted with great advantage for the supply of works for the use

of distant countries possessing the same language. For a single

copy might be printed off with transfer ink, and thus an English

work, for example, might be published in America from stone,

whilst the original, printed from moveable types, made its

appearance on the same day in England.

 

102. It is much to be wished that such a method were

applicable to the reprinting of facsimiles of old and scarce

books. This, however, would require the sacrifice of two copies,

since a leaf must be destroyed for each page. Such a method of

reproducing a small impression of an old work, is peculiarly

applicable to mathematical tables, the setting up of which in

type is always expensive and liable to error, but how long ink

will continue to be transferable to stone, from paper on which it

has been printed, must be determined by experiment. The

destruction of the greasy or oily portion of the ink in the

character of old books, seems to present the greatest impediment;

if one constituent only of the ink were removed by time, it might

perhaps be hoped, that chemical means would ultimately be

discovered for restoring it: but if this be unsuccessful, an

attempt might be made to discover some substance having a strong

affinity for the carbon of the ink which remains on the paper,

and very little for the paper itself.(2*)

 

103. Lithographic prints have occasionally been executed in

colours. In such instances a separate stone seems to have been

required for each colour, and considerable care, or very good

mechanism, must have been employed to adjust the paper to each

stone. If any two kinds of ink should be discovered mutually

inadhesive, one stone might be employed for two inks; or if the

inking-roller for the second and subsequent colours had portions

cut away corresponding to those parts of the stone inked by the

previous ones, then several colours might be printed from the

same stone: but these principles do not appear to promise much,

except for coarse subjects.

 

104. Register printing. It is sometimes thought necessary to

print from a wooden block, or stereotype plate, the same pattern

reversed upon the opposite side of the paper. The effect of this,

which is technically called Register printing, is to make it

appear as if the ink had penetrated through the paper, and

rendered the pattern visible on the other side. If the subject

chosen contains many fine lines, it seems at first sight

extremely difficult to effect so exact a super position of the

two patterns, on opposite sides of the same piece of paper, that

it shall be impossible to detect the slightest deviation; yet the

process is extremely simple. The block which gives the impression

is always accurately brought down to the same place by means of a

hinge; this spot is covered by a piece of thin leather stretched

over it; the block is now inked, and being brought down to its

place, gives an impression of the pattern to the leather: it is

then turned back; and being inked a second time, the paper

intended to be printed is placed upon the leather, when the block

again descending, the upper surface of the paper is printed from

the block, and its undersurface takes up the impression from the

leather. It is evident that the perfection of this mode of

printing depends in a great measure on finding some soft

substance like leather, which will take as much ink as it ought

from the block, and which will give it up most completely to

paper. Impressions thus obtained are usually fainter on the lower

side; and in order in some measure to remedy this defect, rather

more ink is put on the block at the first than at the second

impression.

 

Of copying by casting

 

105. The art of casting, by pouring substances in a fluid

state into a mould which retains them until they become solid, is

essentially an art of copying; the form of the thing produced

depending entirely upon that of the pattern from which it was

formed.

 

106. Of casting iron and other metals.—Patterns of wood or

metal made from drawings are the originals from which the moulds

for casting are made: so that, in fact, the casting itself is a

copy of the mould; and the mould is a copy of the pattern. In

castings of iron and metals for the coarser purposes, and, if

they are afterwards to be worked even for the finer machines,

the exact resemblance amongst the things produced, which takes

place in many of the arts to which we have alluded, is not

effected in the first instance, nor is this necessary. As the

metals shrink in cooling, the pattern is made larger than the

intended copy; and in extricating it from the sand in which it is

moulded, some little difference will occur in the size of the

cavity which it leaves. In smaller works where accuracy is more

requisite, and where few or no after operations are to be

performed, a mould of metal is employed which has been formed

with considerable care. Thus, in casting bullets, which ought to

be perfectly spherical and smooth, an iron instrument is used, in

which a cavity has been cut and carefully ground; and, in order

to obviate the contraction in cooling, a jet is left which may

supply the deficiency of metal arising from that cause, and which

is afterwards cut off. The leaden toys for children are cast in

brass moulds which open, and in which have been graved or

chiselled the figures intended to be produced.

 

107. A very beautiful mode of representing small branches of

the most delicate vegetable productions in bronze has been

employed by Mr Chantrey. A small strip of a fir-tree, a branch of

holly, a curled leaf of broccoli, or any other vegetable

production, is suspended by one end in a small cylinder of paper

which is placed for support within a similarly formed tin case.

The finest river silt, carefully separated from all the coarser

particles, and mixed with water, so as to have the consistency of

cream, is poured into the paper cylinder by small portions at a

time, carefully shaking the plant a little after each addition,

in order that its leaves may be covered, and that no bubbles of

air may be left. The plant and its mould are now allowed to dry,

and the yielding nature of the paper allows the loamy coating to

shrink from the outside. When this is dry it is surrounded by a

coarser substance; and, finally, we have the twig with all its

leaves embedded in a perfect mould. This mould is carefully

dried, and then gradually heated to a red heat. At the ends of

some of the leaves or shoots, wires have been left to afford

airholes by their removal, and in this state of strong ignition a

stream of air is directed into the hole formed