Page:The New International Encyclopædia 1st ed. v. 06.djvu/516

* DRAWING. 448 DRAWING. dip frround. requirinj; tlic use of the line, but per- iiiittiiifi broad, soft strokes and stumped or rubbed-in sliading. Very efTeetive drawings are made liy using a tinted paper — gray or pale blue — on whieli the high lights are laid on in white with chalk or Chinese while, and the darker shades and masses with the pencil, while the tone of the paper is left to represent the intermediate values. The great masters of the Renaissance, huking the 'lead' or plumbago pencil, which is a modern invention, used the lead point or a silver jioint on ])archnient or heavy paper, giving pale gray line: or more often, sanguine or red chalk. The quill took the place of the modern steel pen, making a softer and broader stroke. Instkimental Dn.wviXG. For scientific pur- poses, for the working drawings from which buildings and niachiuery are to be made and erected, and for all purposes requiring great ex- actitude of lepresentation, a ditrerent sort of delineation is necessary from that of the artist working with free hand. Free-hand drawings are personal inter]netations of visible form; in this lies their charm. Their value is artistic, not scientific: no two drawings of the same object by different persons, or by the same person at difTer- cnt times, can be absolutely alike. In mechanical or iiifstniiiiriilal drawing, on the other hand, mathematical exactness of line and dimension is secured by the use of various instruments, the most important of these being the T-square, rulp, and triangles, used in drawing right lines; com- passes for drawing circles and arcs of circles: dividers for laying off exact distances, the scale for determining and measuring dimensions, the protractor for laying off angles, and the ruling- pen for drawing clean and faultless lines in ink: besides a considerable numl>er of special appli- ances which cannot here be enumerated. The ob- ject of drawings executed by these means is not to present the aspects of objects as we see them, but to furnish scientifically correct graphic rec- ords of the actual jjroportions and form-relations of objects, iisually at a much reduced scale. This is effected by means of projections upon imagin- ary vertical and horizontal planes, called plmttK of projection, two dimensions of the object being shown in each projection. These projections com- prise plans or top views, showing the object as if seen from an infinite height above; derations or front. si<le, and rear views, as if seen from a point infinitely distant horizontally: and sections, which siiow the object as if sliced in two and the nearer half removed, exposing the interior struc- ture. These i)rojections show the correct geo- metrical relations of the various dimensions and jiarts of the structure or object, and by the isp of two or more projections all its dimensional relations are exhibited. To assist the eye in in- terpreting these highly conventional drawings — which are really not pictures, but diagrams — they are often (especially in architectural draw- ings) made with the shadows cast as if by sun- light, falling on the object in each projection at an assumed and uniform angle. This angle is usually so taken that the rays, falling from in front of the vertical plane, downward to the right, are projected at 4.'>° to the ground line on eillier projection. The delineation of these con- ventional shadows is n branch of descriptive geon)elr>'. and has received the name of sciot)- raplni ('shadow writing'!. Instrumental drawing is divided, according to its various applications, into mechanical drawing, a general term for all engineering draughting; machine drawing or the preparation of the work- ing drawings for the construction of machinery; architectural drawing, ship draughtinij, topo- graphical ilrawiug, etc.; and the various purely mathematical divisions of dcscriptirc (jroinctrii, stcrcutouiij or the preparations of drawings for cut stonework, criistalloiiruphii. and the like. In architectural drawing and in most forms of machine drawing and engineering, the jmrpose in view is to furnish diagrams of all parts of a structure or machine to lie erected, drawn to exact scale and jnarked with the proposed dimensions, for the guidance of those who are to execute the work. For this jnirpose there are ])repared as many plans as may be necessary, elevations of the various faces of the structure, and sections show- ing its internal arrangement. These are all drawn to a small scale, which may he from one- thirty-second of an inch 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). Many 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, stono-yards, and shops where much of the work is executed, these drawings are supplemented by shop drauiniis 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 shipbuilders, each rib l)eing drawn out to its full size on the smooth lloor of the loft. The drawing of decora- tive details of 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. Perspective Dr.wixo. Perspective drawing stands midway between free-hand or pictorial drawing and instrumental drawing, since it aims to represent the actual aspect of an object from a given point of view, and yet to do this is a mat- ter less of personal and artistic interpretation than of scientific determination. The object ia shown with all the angular distortion and fore- shortening 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- cesses, and not by mere visual impressions. It thus forms a department of descriptive geometry, but the object is rei>rcsented. not as projected by parallel lines on two planes of projection, but as projected, by rays converging to the eye, upon an assiimed picture-plane (or rarely upon a cylin- drical or even spherical surface) intersecting these rays and represented by the paper. The picture is absolutely eorrcct only for the eye placed at the given |>oint of view. .Ml parallel lines not parallel to this plane are shown con- verging to points called vanishing points, and jiarallel planes in like manner converge toward vanishing lines or traces. These are all deter- mined by rules and processes too intricate to bo here detailed. (See PEBSPErTlvE.) There is also a form of .ingular projection called isometric pro- jection, whieli shows all three dimensions at once.