PRINTING. 519 



by means of inking balls, made of leather, firmly stuffed with wool, 

 and provided with handles ; or else by means of elastic rollers. The 

 paper is then laid on, and strongly pressed ; and thus the printing is 

 executed, on one side of the sheet at a time. A single copy is first 

 printed, called the proof; by reference to which, any errors in the 

 composition are corrected, before any more impressions are taken. 



In the process of stereotype printing, a page of moveable types is 

 set up, as already explained, and then covered with a paste or cream 

 of calcined gypsum, freshly mixed with water; which soon hardens, 

 and being then removed, forms a mould, from which the whole page 

 may afterwards be cast, in one solid piece. The common printing 

 press, consists essentially of a frame, supporting a bed, or firm table,, 

 on which the form, containing the types is placed ; and a platten, or 

 strong metallic plate, which, being moved by a lever, on the principle 

 of the screw or inclined plane, presses the paper hard against the 

 types, to take the ink therefrom. In the printing machine, or power 

 press, the types are inked by means of rollers ; the paper supplied 

 by passing over revolving cylinders ; and in some cases the types 

 themselves are arranged around a cylinder, which turns as it receives 

 the paper. 



3. The art of Engraving, presupposes an acquaintance with 

 Drawing, and Perspective ; but we shall here treat only of its me- 

 chanical processes ; reserving those auxiliary studies for the follow- 

 ing chapter, to which they naturally belong. Xylography, or 

 engraving on wood, is performed by first making a drawing of the 

 intended design, on a flat fine grained block, of boxwood in pre- 

 ference, cut across the grain, and planed very smooth. Those parts 

 which are to make no impression, or leave the paper white, are then 

 cut away with a graver, a kind of chisel, pushed forward by the 

 hand ; while those parts which are to receive the ink, are left pro- 

 jecting, as in common types. Sometimes, an engraving is made at 

 once on soft metal ; but more frequently, plaster casts are made from 

 the wood ; and in this way as many metallic copies, like stereotype 

 plates, may be founded, as are desired, for use in common printing. 



Copper plate engraving, differs from that on wood, in having the 

 parts that are to retain the ink and blacken the paper, cut into the 

 surface, instead of being raised above it. The ink, applied by means 

 of a roller, at first covers the whole plate ; but it is then wiped off 

 from the smooth surface, before the paper is applied, which, by severe 

 pressure, in the roller press, receives the ink from the sunken part, 

 in the process of copper-plate printing. Line engraving, is per- 

 formed by means of the graver, already mentioned, or the dry 

 point, of steel, made hard and sharp. Dot engraving, or stippling, 

 is performed by pricking the plate, either with the dry point, which 

 produces circular dots, or with the graver, which makes them triangu- 

 lar. Etching, is executed by corroding the metal with an acid, instead 

 of cutting it away mechanically. The whole copper plate is first 

 covered over with a resinous varnish ; which is then scraped off 

 from those parts which are to be bitten, or eaten away by the acid. 

 Any part, after being slightly bitten, may be stopped out, by washing: 



