I 



MODELLING, PERSPECTIVE, ENGRAVING, PRINTING. 87 



Lahire, Hist, et Mem. ix. 405, 431, 464. Reynolds's Discourses, Burnet's ed. 

 4to, 1842. Cooper on the Painting of the Ancients, Manch. Mem. iii. 510. Ra- 

 phael Mengs Obras, 4to, Madrid, 1780. In Italian, by D'Arezza. Wincklemann, 

 Histoire del' Art chez les Anciens (Trad, de VAllemande), 3 vols. 4to, Paris, 1790- 

 1803. Burnet' s Hints on Composition, 4to, 1827. Lanzi, Storia Pittorica, Trans, 

 by Roscoe, 1828. Rosini, Storia della Pittura Italiana, 4 vols. 8vo. already pub- 

 lished, with plates fol. 



Writing. Nouveau Traite de Diplomatique, 6 vols. 4to, Paris, 1750-65. Butt- 

 ner on the Alphabets of all Nations, Nov. Com. Gott. 1776, p. 106. Astle's Origin 

 and Progress of Writing, 4to, Lond. 1803. 



Measuring Instruments, 8?c. Bion on Math. Insts. 1723, and 4to, Paris, 1752. 

 Adams's Essays, Lond. 1797. Smeaton on the Graduation of Insts. Ph. Tr. 1786, 

 p. 1. Ludlam do. 4to, 1786. Gounella do. Pistoia, 1816. 



Modes of obtaining a Standard, fyc. Condamine on an Invariable Measure, 

 Hist, et Mem. de Paris, 1747, p. 489, H. 83. Remarks on Experiments with 

 Pendulums. Nich. Journ. iii. 29. 



Comparison of Measures. Comparison of French and English Measures, Ph. 

 Tr. 1742, p. 185. Of English Standards, Ph. Tr. 1743, p. 541. Gray on the 

 Measures of Scotland, Ed. Ess. i. 200. Shuckburgh on a Standard of Weights and 

 Measures, Ph. Tr. 1798, p. 133. Kater, Ph. Tr. 1818-19-21. Hall and Foster, 

 Ph. Tr. 1823. Sabine, Ph. Tr. 1828-29. Baily, Report on the New Standard Scale 

 of the Astronomical Society, Trans. Royal Astr. Soc. 1836, ix. See also Lect. XII. 



LECTURE XI. 



ON MODELLING, PERSPECTIVE, ENGRAVING, AND PRINTING. 



WE have examined the principal instruments and materials employed 

 for drawing and for measuring ; we are now to consider, first, the methods 

 of copying solids, and of projecting their images 011 a plane surface ; and 

 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 

 example of a statue, we must determine the situation of a sufficient num- 

 ber 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 part 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 plaster of Paris, since the clay would crack and 

 change its form in drying. This mode of copying by means of plaster is 

 exceedingly 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, form a mould for the ultimate cast. Sometimes a small 



