The tints for imitating the light washes of colour in water-colour drawings and similar work, are produced in various ways, on the principle of covering the stone with a fatty substance in such a manner that it would roll up of full strength all over, except where part had been removed. When chalk is to be imitated in the tints, the stone must be coarse, but very sharp-grained, and the set-off must be made clearly visible throughout the operation. The set-off being made on the grained stone, the margin and high lights are stopped out with gum and acid, and the ground is laid. Should the set-off be too weak, an impression dusted with red chalk or vermilion may be registered upon it, and lightly pressed. The ground forming the tint must be hard enough to bear scraping without smearing, must roll up solidly after etching, and must be removable by solvents of fats. Of the several substances that may be used, preference is given to Brunswick-blac.k and copal-varnish. The evenly laid ground is left to dry, and the lights are put in by the scraper and preserved by etching. In producing tints of various gradations, the ground must not be laid on with the roller, as in the preceding case, but by warming the stone, and applying " rubbing-in " ink till the grain is filled in, then removing the excess of ink by a piece of woollen. The whole subject of tints, and the pigments and driers employed to produce them, is one of great intricacy, and demands unusual care and experience. The best works on the subject will be found quoted in the bibliography at the end of this article.
Mention may here be made of a machine and process designed by Arthur Rigg for doing away with the necessity for having a separate stone for each colour, at least in the case of such work as map-outlines. These outlines are laid down on a thin sheet of brass, and the vazious colours, placed side by side in cakes in an iron frame, are cut apart by scissors. Each piece of brass is then laid upon a cake of the required colour, and made to adhere to it, and the colour is cut to the exact foun of the plate by a small wire-band saw. When all the pieces are furnished with colour, the whole is put togethcr in the chase, and welded into homogeneity by warmth and pressure. The coloured ground-work being printed, an impression from the black " key-stone " is made on the top.
The chromo-lithographic machine made by G. lltrann & Co., Leeds, and in most general use in this country, as well as largely on the Continent, is shown in Fig. 1167.
Type-writing.—During the last 25 years, a great number of machines have been introduced for writing by means of type-letters. The following are the most notable examples.
J. Pratt's (of Alabama) has the type fitted on the face of a small plate, about in. sq. This is supported vertically before a frame carrying the paper by an arrangement of levers capable of giving it both vertical and horizontal motion. The plate is thus shifted into any position, and any type required can he brought opposite the point where the impression is taken. As this is done, the same mechanism which moves the plate sets in motion a small hammer, which strikes the paper on the opposite side to the plate, and forces it against the type, thus producing an impression. When the carbon paper is used, several copies can be taken. In order to limit the number of keys required to operate the levers for shifting the type-plate, there is one set of keys for giving the vertical movements, and another for giving the horizontal movements. Thus, for each letter it is required to move two keys ; but as each key of one set could be used with each key of the other set, a much smaller number of keys is necessary than if one key were requisite for every letter. It may make it appear clearer to say that the depression of one key brings into position the vertical line containing the type wanted, and the horizontal line containing the same type ; consequently the intervention of the two lines, where the special letter needed is to be found, is brought into the proper place for the iinpression. The paper is carried in a small frame,
traversed after each letter by a rachet-wheel and pall. At the end of a line, the frame is raised by a rack worked by a separate key, and at the same time thrown back to the proper side of the machine for commencing a fresh line.
Sir C. Wheatstone invented several type-writing machines. No. 1 is in its main features some what like Pratt's. The paper is held in a vertical frame sliding in guides sense the machine, and the impression iis produced by the blow of a small hammer on the type, the paper being interposed between the hammer and the type, and the type moved to its proper place each time. The differences, however, in the methods of mounting the types and actuating the mechanism are very great. The type is set in three rows on the periphery of a small wheel on a vertical spindle. Thus, by shifting the wheel up or down (in the direction of its axis) the line containing the letter required is brought to the proper point on a level with the hammer, and by rotating the wheel, the letter itself is brought round. Both these motions are effected by depressing the same key, there being a key corresponding to each letter on the type-wheel. The spindle of the type-wheel is rotated by a spring, which gives the traverse to the paper, and whioh therefore is wound up by drawing the frame back to its original position after the completion of each line. The depression of the key, aoting through an arrangement of levers, raises the type-wheel to the required height, and, at the same time, sets free a oatch, so that the wheel is turned round hy the spring till it is caught by the second eatch, the position of which corresponds with that of the letter required. The hammer is worked by an arrangement something like that used in the piano, and so mounted that the blow is given after the type has been brought into position. The action is somewhat heavy, and unless the key is depressed sharply, Boa with a little jerk, the hammer is not operated, and consequently no impression results. But the machine as it stends is capable of very satisfactory work. The other three forms are on a different principle, but mueh alike among themselves. The types are set eaoh on the end of a small lever, and these levers are arranged side by side in the form of a quadrant, each lever being pivoted freely. The whole set of levers is connected with a eliding plate, by shifting which to the right or left a suitable distance, any one of the levers is brought over the point where the impression is to be taken. The type being thus held over the proper spot, a small hammer strikes it down on the paper below. The sliding plate extends along the front of the instrument parallel with the row of keys, which resemble piano-keys. It has in it a number of slots, into eaeh of whioh a pin slides, the arrangement being such that each pin and slot moves the plate a certain distance, differing from that to which any other pin moves it, and corresponding with the distance necessary to bring the letter reqnired into action. Each pin is actuated by one of the keys of the key-board. Thus, by depressing the proper key, any required letter is brought under the hammer. The types are charged with ink by being brought against an inking-pad at each side, as they move. The paper on which the impression is to be received is fitted on a cylinder, which ie revolved continuously. The writing is consequently carried round and round the eylinder. As soon as it reaches the spot where it began, the cylinder is shifted longitudinally, so that a fresh line is commenced. There seems no reason why reciprocating plate, with suitable means for shifting it at the end of the line, should not be substituted for the obviously inconvenient cylinder. With this improvement, perhaps, a metchine of this character might be brought into practical use. In its present form, the arrangement which prevents the use of paper of any other than a certain eize and shape is a considerable drawback.