92 LECTURE XI. 



This machine, like the dividing engine, is sometimes adjusted by the revolu- 

 tions of a screw, and sometimes by the oblique motion of a triangular 

 slider. Besides the cutting graver, which is of a prismatic form, terminated 

 by an oblique surface, other instruments are occasionally employed ; the 

 dry needle makes a very fine line, and leaves the metal that it has displaced 

 to be rubbed off by another tool. Sometimes a number of detached exca- 

 vations are formed by a pointed instrument, and the projections are after- 

 wards removed ; this is called stippling. A burnisher and some charcoal 

 are required for erasing the strokes of the graver, when it is necessary, and 

 for polishing the surface. It is seldom, however, that a plate is begun and 

 completed by dry engraving only. 



For engraving in mezzotinto, the plate is roughened, by scraping it in 

 every direction with a tool made for the purpose, so that an impression 

 from it, in this state, would be wholly dark ; the lights are then inserted, 

 by removing the inequalities of the surface, in particular parts, by means 

 of a smooth scraper and a burnisher. As the plate wears in printing, some 

 of these parts are liable to have the grain a little raised again, so that the 

 lights are less clear in the later impressions than in the proofs. It is well 

 known, that in common engravings the proofs are usually the darkest 

 throughout. 



The most expeditious and most generally useful mode of working on 

 copper, is the process of etching. The plate, being covered with a proper 

 varnish, is usually blackened with smoke, and the drawing is placed on it, 

 with the interposition of a paper rubbed over with red chalk, which, when 

 the drawing is traced with a wooden point, adheres to the varnish, in the 

 form of the outline : or if it is required that the ultimate impression be 

 turned the same way as the drawing, an intermediate outline must be 

 procured in the same manner on a separate paper, and then transferred to 

 the plate. All the outlines thus marked are traced with needles, which 

 make as many furrows in the varnish, and leave the copper bare : the 

 shades are inserted with the assistance of the ruling machine, wherever 

 parallel lines can be employed. The plate thus prepared, and furnished 

 with an elevated border of a proper consistence, is subjected to the action 

 of the diluted nitric acid, until all the parts are sufficiently corroded, care 

 being taken in the mean time to sweep off the air bubbles as they collect, 

 and to stop out, or cover with a new varnish, the lighter parts, which are 

 soonest completed. When the varnish is removed, the finishing touches 

 are added with the graver : and if the plate requires further corrosion, the 

 varnish may sometimes be replaced, without filling up the lines, by apply- 

 ing it on a ball or cushion, taking care to avoid any oblique motion. It is 

 said that the acid sometimes operates so as to undermine the metal on each 

 side, and to render the furrows wider as they become deeper, and that for 

 this reason in etchings, as well as in mezzotintos, the latter impressions 

 are sometimes darker than the proofs ; but this is by no means universally 

 true. It is well known to chemists, that glass may be corroded in a similar 

 manner by means of the fluoric acid. 



An etching may also be expeditiously executed by using a varnish 

 mixed with mutton fat, and drawing upon a paper laid on the plate ; the 



