Instrinuei tal drawing is divided, according to its various application., into mechanical drawing, a general term for all engineering &alighting; machine drawing or the preparation of the work ing drawings for the construction of machinery; arch fleet am/ drawing, ship r/ra ity/i fay, topo graphical drawing, etc.; and the various purely mathematical divisions of th seri pl irc groin( try, sten ()fumy or the preparation, of cut stonework, ermtallography, and the like.
In archiwutural drawing and in most forms of machine drawing and engineering, the purpose in view is to furnish diagrams of all parts of :t strueture Or machine to he erected, drawn to exact scale and marked with the proposed dimensions, for the guidance of those who :ire to execute the work. For this purpose there are prepared as litany plan, as May he necessary, elevations of the various fates of the structure, and sections show ing its internal arrangement. The:, are all drawn to a small scale, which may be from one• thirty-second of an ineli to the foot to one-quarter or even one-half of an inch to the foot (with ap proximately corresponding scales where the met ric system is used). Nally of the details of con struction are shown in drawings to a larger scale, one foot being represented by three-quarters of an inch, one inch, or even two or three inches; and there are also prepared a large number of draw ings of structural details of the full size of the intended work. At the mills, stone-yard:, and shops where much of the work is executed, these drawing: are supplemented by shop drawings pre pared by the several contractors to assist in lay ing out the work, and to serve as exact patterns for its execution. The largest shop drawings are those made in the mold-lofts of the ship-builders, each rib being drawn out to its full size on the smooth floor of the loft. The drawing of decora tive details 14 carving. inlay, and other orna ment in architectural work is of necessity chiefly free-hand work, and involves an artistic element not called for in the purely scientific drawings above described.
P•aseE•rivE DaAwING. Perspective drawing stand: midway between free-hand or pictorial drawing and instrumental drawing. since it aims 1_41 represent the actual aspect of an object from it given point of view. and yet to do this is a mat ter of personal and artistic interpretation than of scientific determination. The object is shown with all the angular distortion and fore which it exhibits to the eye placed at the given point of view; but the exact angles, dimensions. distortions. and foreshortening of
each part are determined by mathematical pro• and not by mere visual impressions. It thus forms a department of descriptive geometry. but the objeet is represented. not its projected by parallel lines on two planes of but as projected, by rays converging t41 the eye, upon an assumed picture.plane for rarely upon a cylin drical or even ,plierieal surface) intersecting these rays and represented by the paper. The picture is absolutely correct only for the eye placed at. the given point of VIM. All parallel line, not parallel to this plane are shown con ‘erging to points called vanishing points, and parallel planes in like 111.11111er converge toward ‘anishing lines or tracts. These are all deler• mined by rules and processes to intricate 111 he here detailed. I See Pm:sett-11\ c.) There is also a form of angular 14owicirie pro jc•tion. \Odell shows all three at once, is does a perspective drawing. bit with a uni form scale for all parts of the picture. while in true perspective the same actual dimension ap pears of varying size according to its distance from the eve.
A perspective drawing. thus scientifically laid out as to its outlines. may be finished. as to line,. ceder, light and shade, and aecessories, in a pic torial and artistic manner, as in Iree-hand draw ing; it then emerges from the category of scien tific drawing into that of tine art. Indeed. no artist can master the correct portrayal of form, especially of scenery and buildings. without train ing in perspective; it is acoirdingly an important branch of study in all schools of art. It is abso lutely indispensable to the scene-painters, and forms the basis of the illusory effects of the stage setting. The artists of the Benaissance were the first to reduce its principles to system and many of them were carried away by the fascination of its possibilities: the later artists, especially the Venetians (notably Pao'o Veronese and later Tiepolol, and the mighty .)Tichelangelo, employed its resources with extraordinary effect in their great decorative paintings. .Japanese drawings are interesting for their treatment of perspective, the point of view being in almost all eases as sumed at a high elevation, giving an approxima tion to the effect of what is called 'bird's-eye per 6pect lye.'