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LECTURE XL 



ON MODELLING, PERSPECTIVE, ENGRAVING, Aljijy 



PRINTING. 



\y E have examined die principal instrupicnts and materials employed for 

 <lra>yingand for measuring; we ^ve now to consider, fust, the methods of co- 

 pying solids, and of projecting their images on a plane surface ; 5ind secondly, 

 the arts of perpetuating the works of the pen and of the pencil by engraving 

 and printing. 



When it is required to make a copy of a solid >of an irregular form, as, for 

 .^xample, of a sta.tue, we jnust xletexmine the situation of a sufficient number 

 of points to guide uS in our work with accuracy, by means of an instrument 

 ■capable of being fixed in any required situation, so that the extremity of a 

 sliding bar, or pin, may be in contact with each point in the original, and 

 then removed to a similar pait of another frame, on which the copy is placed, 

 a perforation being made, by degrees, in the block, so as to suffer the pin to 

 arrive at its proper place, at which it stops. (Plate VII. Fig. 98.) 



The model of a statue is generally first made of clay, and a cast of this 

 taken immediately in nlaster of Paris, since the clay would crack and change 

 its form in drying. This mode of copying, by means of plaster, is exceed- 

 ingly useful in various departments of the mechanical arts: the original is 

 well oiled and placed in a proper vessel ; a mixture of prepared plaster and 

 water, of the consistence of cream, is then poured on it; this in a short time 

 hardens, and is divided into several parts, in such a manner as not to injure 

 the original figure in its removal. These pieces, being again united, fornv a 

 mould for the ultimate cast. Sometimes a small figure is first modelled in a 

 mixture of wax, turpentine, and oil; and a mould being formed on this, 

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