71 



LECTURE X. 



ON DRAWING, WRITING, AND MEASURING. 



HAVING investigated all the general principles and laws of motion, and 

 of mechanical power, we may now proceed to the consideration of parti- 

 cular departments of practical mechanics. But before we can satisfactorily 

 compare the various forces which we are to employ or to oppose, we must 

 have some mode of determining their magnitude ; and we must begin by 

 examining the spaces which are measures of their action : a knowledge of 

 the instruments employed for delineation, and of the rules of perspective 

 projection, is also necessarily required as a previous step in the study of 

 practical mechanics. We have therefore to consider, as preliminary 

 subjects, first, the arts which may be expressed by the terms instrumental 

 geometry, or the geometry of mechanics : secondly, statics, or the mode 

 of ascertaining the magnitude of weights and of other active forces ; and 

 thirdly, the examination of the passive strength of materials of various 

 kinds, and of the negative force of friction. 



The art of drawing can scarcely be distinguished by any correct defini- 

 tion from painting. In its simplest state, when we merely imitate an 

 original laid before us, it is called copying ; and in writing, we only copy 

 the letters of the alphabet. If we proceed in a mathematical manner in 

 the operation of drawing, we require a number of geometrical instruments, 

 which are still more necessary for the first construction of diagrams or 

 figures. In modelling and sculpture, a solid is simply imitated ; but when a 

 solid is represented on a plane, the principles of perspective are employed in 

 determining the position of the lines which are to form the picture. The 

 productions of the arts of drawing and writing are multiplied and per- 

 petuated by means of engraving and printing ; inventions which have been 

 the sources of inestimable advantage in the instruction and civilisation of 

 mankind. 



In drawing, we may employ the pen, the pencil, chalks, crayons, inks, 

 water colours, or body colours ; we may paint in miniature, in distemper, 

 in fresco, in oils, in varnish, in wax, or in enamel ; and we may imitate 

 the effects of painting by mosaic work or by tapestry. 



The first step in copying a drawing or in painting, is to procure a correct 

 outline : a master of the art can do this with sufficient accuracy, by such 

 an estimate of the proportions of the figures as the eye alone enables him 

 to form ; especially if he be assisted by lines which divide the original into 

 a number of squares, and enable him to transfer their contents to the corre- 

 sponding squares of the copy, which may in this manner be reduced or 

 enlarged, when it is required. But a copy may sometimes be more expe- 

 ditiously made by tracing immediately from the original, when the mate- 

 rials employed are sufficiently transparent to admit the outlines to be seen 

 through them ; or, where the original is of no value, by pricking a number 





