LITHOGRAPHY 131 



The machine is shown -with its tympan down, ready for starting ; this is effected by 

 pressing lightly iipon the lever, b, which raises a catch, and allows the weight, M, to 

 descend in the direction of its present inclination, and act upon the connections with 

 the striking-forks, so as to bring one of the bands upon the fast pulley, B, and make 

 the scraper and its frames move forward. The return is caused by the frame, F, 

 coming in contact with a stop, c, which, yielding, acts upon the striking-forks by its 

 bar, d, upon which it may be adjusted to give the travel required. On the return 

 being accomplished, the machine stops itself by a striking action against stop e, the 

 catch, b, falling in to prevent the weight descending to its full throw, and thus retain- 

 ing the two bands upon the two dead pulleys, A, c, while the machine is prepared for 

 another impression. 



The action of the scraper is peculiar and novel : it is balanced so that its tendency 

 is to remain slightly raised, but in its forward movement, and at the point desired, it 

 is made to descend by a stop fixed upon the top of the main standard, L, into a posi- 

 tion vertical, or nearly so, in -which position it is retained by its own onward progress 

 against strong abutments projecting from the frames, F ; on the return it resumes its 

 raised position, and passes brack without impediment. The scraper may be adjusted 

 to give the pressure desired, or the table on which the stone is placed regulated by 

 screws. 



The advantages embodied in this machine will be at once recognised by those inte- 

 rested. The pulling down of the scraper, and the labour a.nd inconvenience attendant 

 upon that operation, are entirely superseded by the simple and effectual valve-like 

 movement just explained, which forms the groundwork of this combination, although 

 it will alike apply to the press-work by hand, and is the most striking novelty in tho 

 machine. 



LITHOGRAPHY. Though this subject belongs rather to the arts of taste and 

 design than to productive manufactures, its chemical principles fall within the pro- 

 vince of this Dictionary. 



The term lithography is derived from \idos, a stone, and ypaQ-fi, writing, and desig- 

 nates the art of throwing off impressions upon paper of figures and writing previously 

 traced upon stone. The processes of this art are founded 



1. Upon the adhesion to a grained or smoothly-polished limestone of an encaustic 

 fat which forms the lines or traces. 



2. Upon the power acquired by the parts penetrated by this encaustic of attract- 

 ing to themselves, and becoming covered with, a printer's ink having linseed-oil for 

 its basis. 



3. Upon the interposition of a film of water, which prevents the adhesion of the ink 

 in all the parts of the surface of the stone not impregnated with the encaustic. 



4. Lastly, upon a pressure applied to the stone, such as to transfer to paper the 

 greater part of the ink which covers the greasy tracings or drawings of the encaustic. 



The lithographic stones of the best quality are still procured from the quarry of 

 Solenhofen, a village at no great distance from Munich, where this mode of printing 

 had its birth. They resemble in their aspect the yellowish-white lias of Bath, but 

 their geological place is much higher than the lias. Abundant quarries of these fine- 

 grained limestones occur in the county of Pappenheim, along the banks of the Danube, 

 presenting slabs of every required degree of thickness, parted by regular seams, and 

 ready for removal with very little violence. The good quality of a lithographic stone 

 is generally denoted by the following characters : its hue is of a yellowish-grey, and 

 uniform throughout ; it is free from veins, fibres, and spots ; a steel point makes an 

 impression on it with difficulty ; and the splinters broken off from it by the hammer 

 display a conchoidal fracture. 



The Munich stones are retailed on tho spot in slabs or layers of equal thickness ; 

 they are quarried with the aid of a saw, so as to sacrifice as little as possible _of the 

 irregular edges of the rectangular tables or plates. One of the broad faces is^then 

 dressed, and coarsely smoothed. The thickness of these stones is nearly proportional 

 to their other dimensions ; and varies from 1 inch to 3 inches. 



In each lithographic establishment the stones receive their finishing, dressing, and 

 polishing ; which are performed like the grinding and polishing of mirror-plate. The 

 work is done by hand, by rubbing circularly a moveable slab over another in a hori- 

 zontal position, with fine-sifted sand and water interposed between the two. The 

 style of work that the stone is intended to produce determines the kind of polish 

 that it should get. For crayon-drawing the stone should be merely grained more or 

 less fine according, to the fancy of the- draughtsman. The higher the finish of tho 

 surface the softer are the drawings ; but the printing process becomes sooner pasty, 

 and a smaller number of impressions can be taken. Works in ink require the stone 

 to be more softened down, and finally polished with pumice and a little water. Tho 

 stones thus prepared are packed for use with white paper interposed between their faces. 



K2 



