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

of threads in a given

length which is copied; the form of the thread, and length as

well as the diameter of the screw to be cut, are entirely

independent of those from which the copy is made. There is

another method of cutting screws in a lathe by means of one

pattern screw, which, being connected by wheels with the mandril,

guides the cutting point. In this process, unless the time of

revolution of the mandril is the same as that of the screw which

guides the cutting point, the number of threads in a given length

will be different. If the mandril move quicker than the cutting

point, the screw which is produced will be finer than the

original; if it move slower, the copy will be more coarse than

the original. The screw thus generated may be finer or coarser—

it may be larger or smaller in diameter—it may have the same or

a greater number of threads than that from which it is copied;

yet all the defects which exist in the original will be

accurately transmitted, under the modified circumstances, to

every individual generated from it.

 

154. Printing from copper plates with altered dimensions.

Some very singular specimens of an art of copying, not yet made

public, were brought from Paris a few years since. A watchmaker

in that city, of the name of Gonord, had contrived a method by

which he could take from the same copperplate impressions of

different sizes, either larger or smaller than the original

design. Having procured four impressions of a parrot, surrounded

by a circle, executed in this manner, I shewed them to the late

Mr Lowry, an engraver equally distinguished for his skill, and

for the many mechanical contrivances with which he enriched his

art. The relative dimensions of the several impressions were 5.5,

6.3, 8.4, 15.0, so that the largest was nearly three times the

linear size of the smallest; and Mr Lowry assured me, that he was

unable to detect any lines in one which had not corresponding

lines in the others. There appeared to be a difference in the

quantity of ink, but none in the traces of the engraving; and,

from the general appearance, it was conjectured that the largest

but one was the original impression from the copperplate.

 

The means by which this singular operation was executed have

not been published; but two conjectures were formed at the time

which merit notice. It was supposed that the artist was in

possession of some method of transferring the ink from the lines

of a copperplate to the surface of some fluid, and of

retransferring the impression from the fluid to paper. If this

could be accomplished, the print would, in the first instance, be

of exactly the same size as the copper from which it was derived;

but if the fluid were contained in a vessel having the form of an

inverted cone, with a small aperture at the bottom, the liquid

might be lowered or raised in the vessel by gradual abstraction

or addition through the apex of the cone; in this case, the

surface to which the printing-ink adhered would diminish or

enlarge, and in this altered state the impression might be

retransferred to paper. It must be admitted, that this

conjectural explanation is liable to very considerable

difficulties; for, although the converse operation of taking an

impression from a liquid surface has a parallel in the art of

marbling paper, the possibility of transferring the ink from the

copper to the fluid requires to be proved.

 

Another and more plausible explanation is founded on the

elastic nature of the compound of glue and treacle, a substance

already in use in transferring engravings to earthenware. It is

conjectured, that an impression from the copperplate is taken

upon a large sheet of this composition; that this sheet is then

stretched in both directions, and that the ink thus expanded is

transferred to paper. If the copy is required to be smaller than

the original, the elastic substance must first be stretched, and

then receive the impression from the copperplate: on removing the

tension it will contract, and thus reduce the size of the design.

It is possible that one transfer may not in all cases suffice; as

the extensibility of the composition of glue and treacle,

although considerable, is still limited. Perhaps sheets of India

rubber of uniform texture and thickness, may be found to answer

better than this composition; or possibly the ink might be

transferred from the copper plate to the surface of a bottle of

this gum, which bottle might, after being expanded by forcing air

into it, give up the enlarged impression to paper. As it would

require considerable time to produce impressions in this manner,

and there might arise some difficulty in making them all of

precisely the same size, the process might be rendered more

certain and expeditious by performing that part of the operation

which depends on the enlargement or diminution of the design only

once; and, instead of printing from the soft substance.

transferring the design from it to stone: thus a considerable

portion of the work would be reduced to an art already well

known, that of lithography. This idea receives some confirmation

from the fact, that in another set of specimens, consisting of a

map of St Petersburgh, of several sizes, a very short line,

evidently an accidental defect, occurs in all the impressions of

one particular size, but not in any of a different size.

 

155. Machine to produce engraving from medals. An instrument

was contrived, a long time ago, and is described in the Manuel de

Tourneur, by which copperplate engravings are produced from

medals and other objects in relief. The medal and the copper are

fixed on two sliding plates at right angles to each other, so

connected that, when the plate on which the medal is fixed is

raised vertically by a screw, the slide holding the copperplate

is advanced by an equal quantity in the horizontal direction. The

medal is fixed on the vertical slide with its face towards the

copperplate, and a little above it.

 

A bar, terminating at one end in a tracing point, and at the

other in a short arm, at right angles to the bar, and holding a

diamond point, is placed horizontally above the copper; so that

the tracing point shall touch the medal to which the bar is

perpendicular, and the diamond point shall touch the copperplate

to which the arm is perpendicular.

 

Under this arrangement, the bar being supposed to move

parallel to itself, and consequently to the copper, if the

tracing point pass over a flat part of the medal, the diamond

point will draw a straight line of equal length upon the copper;

but, if the tracing point pass over any projecting part of the

medal, the deviation from the straight line by the diamond point,

will be exactly equal to the elevation of the corresponding point

of the medal above the rest of the surface. Thus, by the transit

of this tracing point over any line upon the medal, the diamond

will draw upon the copper a section of the medal through that

line.

 

A screw is attached to the apparatus, so that if the medal be

raised a very small quantity by the screw, the copperplate will

be advanced by the same quantity, and thus a new line of section

may be drawn: and, by continuing this process, the series of

sectional lines on the copper produces the representation of the

medal on a plane: the outline and the form of the figure arising

from the sinuosities of the lines, and from their greater or less

proximity. The effect of this kind of engraving is very striking;

and in some specimens gives a high degree of apparent relief. It

has been practised on plate glass, and is then additionally

curious from the circumstance of the fine lines traced by the

diamond being invisible, except in certain lights.

 

From this description, it will have been seen that the

engraving on copper must be distorted; that is to say, that the

projection on the copper cannot be the same as that which arises

from a perpendicular projection of each point of the medal upon a

plane parallel to itself. The position of the prominent parts

will be more altered than that of the less elevated; and the

greater the relief of the medal the more distorted will be its

engraved representation. Mr John Bate, son of Mr Bate, of the

Poultry, has contrived an improved machine, for which he has

taken a patent, in which this source of distortion is remedied.

The head, in the title page of the present volume, is copied from

a medal of Roger Bacon, which forms one of a series of medals of

eminent men, struck at the Royal Mint at Munich, and is the first

of the published productions of this new art.(3*)

 

The inconvenience which arises from too high a relief in the

medal, or in the bust, might be remedied by some mechanical

contrivance, by which the deviation of the diamond point from the

right line (which it would describe when the tracing point

traverses a plane), would be made proportional not to the

elevation of the corresponding point above the plane of the

medal, but to its elevation above some other parallel plane

removed to a fit distance behind it. Thus busts and statues might

be reduced to any required degree of relief.

 

156. The machine just described naturally suggests other

views which seem to deserve some consideration, and, perhaps,

some experiment. If a medal were placed under the tracing point

of a pentagraph, an engraving tool substituted for the pencil,

and a copperplate in the place of the paper; and if, by some

mechanism, the tracing point, which slides in a vertical plane,

could, as it is carried over the different elevations of the

medal, increase or diminish the depth of the engraved line

proportionally to the actual height of the corresponding point on

the medal, then an engraving would be produced, free at least

from any distortion, although it might be liable to objections of

a different kind. If, by any similar contrivance, instead of

lines, we could make on each point of the copper a dot, varying

in size or depth with the altitude of the corresponding point of

the medal above its plane, than a new species of engraving would

be produced: and the variety of these might again be increased,

by causing the graving point to describe very small circles, of

diameters, varying with the height of the point on the medal

above a given plane; or by making the graving tool consist of

three equidistant points, whose distance increased or diminished

according to some determinate law, dependent on the elevation of

the point represented above the plane of the medal. It would,

perhaps, be difficult to imagine the effects of some of these

kinds of engraving; but they would all possess, in common, the

property of being projections, by parallel lines, of the objects

represented, and the intensity of the shade of the ink would

either vary according to some function of the distance of the

point represented from some given plane, or it would be a little

modified by the distances from the same plane of a few of the

immediately contiguous points.

 

157. The system of shading maps by means of lines of equal

altitude above the sea bears some analogy to this mode of

representing medals, and if applied to them would produce a

different species of engraved resemblance. The projections on the

plane of the medal, of the section of an imaginary plane, placed

at successive distances above it, with the medal itself, would

produce a likeness of the figure on the medal, in which all the

inclined