95 



LECTURE X. 



ON DRAWING, WRITING, AND MEASURING. 



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

 of mechanical power, we may now proceed to the consideration of particular 

 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 definitioa" 

 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 draw- 

 ing, we require a number of geometrical instruments, whicli 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 positioa 

 of the lines which are to form the picture. The productions of the arts of 

 drawing and writing are multiplied and perpetuated by means of engraving 

 and printing; inventions which have been the sources of inestimable advantage 

 in the instruction and civilisation of mankind. 



