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 parts of it would be dark in proportion to their inclination. Other species of engraving might be conceived by substituting, instead of the imaginary plane, an imaginary sphere or other solid, intersecting the figure in the medal.
158. Lace made by caterpillars. A most extraordinary species of manufacture, which is in a slight degree connected with copying, has been contrived by an officer of engineers residing at Munich. It consists of lace, and veils, with open patterns in them, made entirely by caterpillars. The following is the mode of proceeding adopted: he makes a paste of the leaves of the plant, which is the usual food of the species of caterpillar(4*) he employs, and spreads it thinly over a stone, or other flat substance. He then, with a camel-hair pencil dipped in olive oil, draws upon the coating of paste the pattern he wishes the insects to leave open. This stone is then placed in an inclined position, and a number of the caterpillars are placed at the bottom. A peculiar species is chosen, which spins a strong web; and the animals commencing at the bottom, eat and spin their way up to the top, carefully avoiding every part touched by the oil, but devouring all the rest of the paste. The extreme lightness of these veils, combined with some strength, is truly surprising. One of them, measuring twenty-six and a half inches by seventeen inches, weighed only 1.51 grains; a degree of lightness which will appear more strongly by contrast with other fabrics. One square yard of the substance of which these veils are made weighs 4 1/3 grains, whilst one square yard of silk gauze weighs 137 grains, and one square yard of the finest patent net weighs 262 1/2 grains. The ladies' coloured muslin dresses, mentioned in the table subjoined, cost ten shillings per dress, and each weigh six ounces; the cotton from which they are made weighing nearly six and two-ninth ounces avoirdupois weight.
Weight of one square yard of each of the following articles(5*)
Weight of
Weight cotton used
Value finished of in waking
per yard one square one square
Description of goods measure yard yard
s. d. Troy grains Troy grains
Caterpillar veils — 4 1/3 — Silk gauze 3-4 wide 1 0 137 — Finest patent net — 262 1/2 — Fine cambric muslin — 551 — 6-4ths jaconet muslin 2 0 613 670 Ladies' coloured muslin dresses 3 0 788 875 6-4ths cambric 1 2 972 1069 9-8ths calico 0 9 988 1085 1/2-yard nankeen 0 8 2240 2432
159. This enumeration, which is far from complete, of the arts in which copying is the foundation, may be terminated with an example which has long been under the eye of the reader; although few, perhaps, are aware of the number of repeated copyings of which these very pages are the subject.