the taste and industry of tnose wno would perpetuate | that covered by the lines of the writing. From these and multiply valuable works of art in the several styles best suited to their respective subjects.

raised wooden lines impressions are taken. This practice is of ancient date in China, and some of those who have bestowed research on the matter are inclined to fix it about A.D. 930. As far as it is now possible to trace the introduction of wood-engraving into Europe, it would appear that the Venetians, in their commerce with the Chinese, early learned this art, and practised it before it was known to other European nations. But the art was eagerly acquired by Germany and the Low Countries, and in 1433, or thereabouts, they carried on a considerable commerce in playing-cards and prints of saints. At that time. the engraver on wood was called Formschneider, or figure-cutter, a term still in use. The little prints of

Steel-engraving is performed in the same way as copper-plate engraving, with the exception of some slight modifications in the use of acids. The improved presses used in both kinds of engraving are due to Mr. Jacob Perkins, who may be said to have established steel-engraving by his invention of decarbonizing the plate, so as to make it fit to be engraven on, and also by his interesting method of multiplying impressions on steel-plates. Mr. Perkins's method of transfer-engraving originated in the transfer processes employed in engraving the copper cylinders used in calico-printing. The subject intended to be multiplied is first engraved either by hand or mecha-saints were rudely executed, and had a great sale nically, or the two may be combined, in the best style of art on a plate of soft steel: the plate is then hardened. A decarbonized steel cylinder is next rolled over the hardened plate by means of powerful machinery, until the engraved impression appears in relief, the hollow lines of the original being raised on the cylinder. The roller is then reconverted to the condition of ordinary steel, and hardened, after which it can be used for returning the impression to any number of decarbonized plates, each of which is of course an exact counterpart of the original. It is said that each of these plates when hardened would give 150,000 impressions without being materially worn. The original plate thus serves only to give one impression to the transfer roller, which in its turn is used to produce any number of plates. Should any accident happen to the transfer roller, another impression can easily be taken from the original plate.

In order to decarbonize the plates they are placed in a vertical position in cast-iron boxes not less than inch thick, and surrounded on all sides by a stratum of iron-filings not less than inch thick; the boxes are then placed in a furnace, and after being heated are cooled very slowly by stopping up all the air passages, and covering the boxes with cinders to the depth of 6 or 7 inches. The decarbonized plates are reconverted into steel by enclosing them in boxes as above, and surrounding them with fine charcoal made from leather; they are left at the proper heat for from 3 to 5 hours, and on being taken out are immediately plunged in a vertical position into cold water.

Wood-engraving (Xylography) is said to have had its origin in China, the birth-place of many other valuable inventions, and to have been due to the peculiar structure of the Chinese language, in the writing of which a separate symbol is used for each idea, and words are not made up, as with us, by a combination of letters. The number of these symbols or characters is therefore so vast that it would be almost impossible to print their books with movable types. Their method of printing is therefore as follows: the work to be printed is carefully transcribed upon transparent paper, only one side of which is written on. The sheets are then glued down upon wooden tablets, and all the wood is cut away except

VOL. I.

among the common people. In Germany they were called Helgen, or Helglein; in France, Dominos. They were at first sold separately, and thus were soon dispersed and lost; but after a time they were pasted into religious books for the sake of preserving them, and thus probably originated the custom of illustrating books with engravings. From that time the art made decided progress. Block-books, as they are called, made their appearance. These were books in which the productions of the wood-engraver were simply collected in the form of volumes, some of which are still extant. One of the earliest, called the Biblia Pauperum, or Bible of the Poor, consists of 40 leaves, small folio, printed from the same number of engraved blocks of wood, on one side of the paper only. These prints are placed two by two, facing each other, so that by pasting their backs the book has the appearance of being printed in the usual way on both sides. The use of printed characters in books is closely connected with the origin of woodengraving, but the exact time of the invention is a disputed point. One of the most generally received accounts of this discovery is thus given by an old German chronicler :

"At this time (about 1438), in the city of Mentz, on the Rhine, in Germany, and not in Italy as some have erroneously written, that wonderful and then unheard-of art of printing and characterising books was invented and devised by John Gutenberger, citizen of Mentz, who, having expended most of his property in the invention of this art, on account of the difficulties which he experienced on all sides, was about to abandon it altogether; when by the advice and through the means of John Fust, likewise a citizen of Mentz, he succeeded in bringing it to perfection. At first they formed or engraved the characters or letters in written order on blocks of wood, and in this manner they printed the vocabulary called a 'Catholicon.' But with these forms or blocks they could print nothing else, because the characters could not be transposed in these tablets, but were engraved thereon as we have said. To this invention succeeded a more subtle one, for they found out the means of cutting the forms of all the letters of the alphabet, which they called matrices, from which again they cast characters of copper or tin of sufficient hardness

RR

Fig. 865 shows the extent to which the points are | usually ground down. Gravers are used for nearly every description of wood-cutting, occasionally, not even excepting "tinting," the technical term applied to cutting series of parallel lines, which, when engraved, form an even and uniform tint. For this process, however, there is a distinct set of tools, Fig. 866, thinner, and ground to a much more acute angle at the face. These tools, though thin, ought to be sufficiently strong at the back to prevent their bending when used: their

steel engraving. In the latter case, the forefinger is extended on the back of the tool, so as to press the point into the plate. (See Fig. 863.) In wood-cutting this is not necessary, but, on the contrary, the force of the hand has to be checked by the thumb, which in small subjects is rested against the side of the block, allowing the blade to move freely, but ever ready to check it in case of a slip. See Fig. 871. In larger subjects, the thumb accomplishes the same ends by resting on the surface of the block. See Fig. 872.

Fig. 872.

Engraving requires delicate and skilful workmanship, and makes large demands on the eyesight as well as on the dexterity of hand of those who practise it. Some parts of the work are generally supposed to require the use of magnifying glasses, and much of it must necessarily be accomplished by lamp-light. The most experienced engravers, however, are slow to recommend the use of glasses to those who can possibly do without their assistance. Young persons commencing the art of wood-cutting seem to imagine that a magnifying glass must of necessity form part of their apparatus. The sort of glass employed is similar to that of watch-makers, and consists of a single lens fitted into a short tube, and rather wider at the part applied to the eye. Such aid should only be sought when sight begins to fail; and even then the glass should at first be of low magnifying power. Various means are employed to protect the eyes from the light, and the face from the heat, of the lamp. One of these consists in the use of a large glass globe filled with water, which is interposed between the lamp and the engraver's block. By the use of these globes one lamp is found sufficient for three or four persons, and each person has a clear and cool light to work by. In damp or frosty weather, the breath of the engraver is apt to injure his work, unless some contrivance be adopted to prevent its playing on the surface of the block. This is usually found in a screen of thin pasteboard or stiff paper, temporarily tied across the mouth and nostrils in such weather. The eyes have their own protection, from a shade which most wood-engravers wear, not only to guard the sight, but also to concentrate the view on the work in hand. Such shades are, however, very objectionable, as they confine hot air close to the eyes, which require for healthy action the free circulation of fresh air.

The pupil in wood-engraving commences with the cutting of parallel lines or tints, straight and waved,

and then proceeds to simple forms in outline, without | engravings can be transferred to stone, and worked by

any shading that is expressed by cross-lines. Such shading is necessarily difficult in a material where all the parts intended to be light have to be cut away, and the dark lines alone remain standing; and consequently in cross-shading the interstices have to be carefully hollowed out, without injury to the lines. | Complicated subjects should be long deferred, and never attempted till decided success has attended the simpler efforts.

power presses, thus lessening the expense, and affording a convenient mode of multiplying maps which may be required in large quantities.

ENVELOPE-FOLDING MACHINE. Among the machines exhibited in motion in the Great Exhibition, Messrs. De la Rue and Company's envelopefolding machine was always surrounded by a crowd of interested spectators; and it is worthy of the admiration which it has excited; for whether we regard the rapidity and precision with which it does its work (folding an envelope every second), or study the anatomy of its structure, we have a striking | illustration of the proposition, that whatever merely mechanical operation is performed well by human hands, can be effected much better and far more quickly by a self-acting machine. Through the kindness of the inventors, we are able to lay before our readers a sufficiently minute description of this machine.

Lithography, or engraving on stone, is a modern invention, ascribed to a musician named Senefelder, connected with the theatre at Munich, about the year 1800. The term engraving is not truly applicable to the process, as usually carried on; but in Germany a great deal of actual engraving on stone with the burin is practised. The results of this method, however, are so greatly inferior to those of copperplate engraving, that it is not likely ever to come into general use. The art of taking impressions from drawings made on stone depends on the readi- As the various motions of this machine are proness with which calcareous stone imbibes water, its duced by means of cams, it will be necessary to the great disposition to adhere to resinous or oily sub-due understanding of the subject to describe the stances, and the disposition those substances have to nature of a cam, and the method by which it is combine, and to repel water or any substance moist-produced. ened with water. Drawings made on a polished surface of calcareous stone with a resinous or oily substance adhere strongly to the stone, and are not at all affected by water poured over it, and which the other parts imbibe readily. But if a resinous or oily body be then passed over the stone, it will adhere strongly to the drawing, and not to the watery parts of the

stone.

The drawings, therefore, in lithography, are made with ink and chalk of a soapy nature. The ingredients of the former are tallow-soap, pure white wax, lampblack, and a small quantity of tallow, all boiled together, and when cool, dissolved in distilled water. The ingredients for the chalk are the same, with a small quantity of potash added during the boiling. After the drawing on the stone is perfectly dry, a very weak solution of sulphuric acid is poured over it, which takes up the alkali from the ink or chalk, and leaves an insoluble substance behind it, while it lowers in a slight degree the surface of the stone not drawn upon, and prepares it for the free absorption of water. Weak gum-water is next applied, to close the pores of the stone, and keep it moist. The stone is then washed with water, and the printing-ink applied in the ordinary way. It then passes through the press; the washing with water and daubing with ink being repeated after every impression. The impressions may be multiplied to a great extent, without any marked failure in the effect. As many as 70,000 copies or prints have been taken from one stone, the last being nearly as good as the first. The work can also be performed with great expedition and economy. Drawings made with the chemical ink on paper prepared with a solution of sise or gum tragacanth can be transferred to the stone, by being merely laid thereon and passed through the press, when the subsequent processes can be carried on, as already described. Copper-plate and steel

A Cam is a contrivance in which sliding contact is employed to communicate a velocity ratio, either constant or varying, between two pieces of mechanism. The extent of motion of one of these pieces must be limited, that of the other may or may not be continuous. Thus for example, if one of them be required to move in a short rectilinear path in the manner of a rack or sliding bar, an axis revolving indefinitely in one direction, or with a defined to and fro rotation in both directions, may, by means of a cam, be made to give motion to this bar or rack. If the relation between the speed of movement of the bar and that of the axis be uniform, then the velocity ratio is said to be constant. If, however, the speed of the bar does not, in the successive periods of its motion, bear any uniform relation to the velocity of the axis, then the velocity ratio of the two pieces is said to be varying.

D

Let A, Fig. 873, be the centre of motion of a plate, to which an alternating movement of rotation can be given. In this plate a slit a b is pierced, having parallel sides, so as to embrace and nearly fit a pin ", carried by a bar CD fitted between guides, so as to be capable of sliding in the direction of its length. While the plate revolves in the direction of the arrow, the inner side of the slit presses against the Fig. 873. pin and moves it further from the centre A; but when the plate revolves in the opposite direction the outer edge of the slit acts against the pin, and moves it in towards the centre. If the linear velocity of translation of the bar be required to be exactly

equal to the circular velocity of a point c of the plate, | (which is called a follower) is shown in Fig. 877. The then the curve a b must be an involute of the circle whose radius is A c. So for other constant velocity ratios, curves can be found which shall fulfil the required conditions.