Page:The American Cyclopædia (1879) Volume VI.djvu/258

 250 DRAWING fonnd will be the orthographic projections of the corners ; and if these points be connected by corresponding lines, there will be outlines of the brick under three views or projections : on the bottom of the box, a rectangle 8 by 4 in., being the plan of the brick; on one side, a FIG. l. rectangle 8 by 2^ in., the side elevation; on the other side, a rectangle 4 by 2 in., the end elevation. If the brick be inclined to either or all of the sides of the box, the projected outlines will be varied ; but the same rule for determining the position of points obtains, viz. : by letting fall perpendiculars on the planes to which they are referred. The or- thographic projection of any object in outline is the shadow it would cast on a plane perpen- dicular to the rays of the sun, if held between it and the sun. Simple objects in general may be defined by two views, a plan and elevation ; but often, to illustrate the construction of the interior, sections are necessary, that is, the appearances that might be presented were the objects cut by planes ; all portions that would be thus absolutely cut are designated by filling up the outline with a quantity of inclined par- allel straight lines, at equal intervals from each other ; should there be distinct parts in section, in contact with each other, to prevent confu- sion the different sections are expressed by lines inclined in opposite directions. In most architectural and mechanical constructions it would be obviously impossible that they could be drawn full size. Scales are therefore made use of in which fractional parts represent wholes. The scale in most common use in architectural drawings is that of one fourth of an inch to the foot, or ^ of the lineal dimen- sions; in mechanical drawings, one fourth or one eighth full size, that is, as usually under- stood, one fourth or one eighth of the lineal dimensions. Working drawings of machines, or those intended to be used in construction, are generally laid off to as large a scale as possi- ble ; they are mostly outline drawings, consist- ing of lines to indicate the form of the object represented. The roundness, fulness, or ob- liquity of the individual surfaces is not indica- ted by the lines, although it may be generally inferred from the relation of the different views of the same part. The direct significance of an outline drawing is often considerably in- creased by strengthening those lines which indi- cate the contours of surfaces resting in the shadow. That all parts may be shade-lined according to one uniform rule, the light is supposed to fall upon the object obliquely at an angle of 45 (that the horizontal and vertical lines may be relieved equally), and in general (fig. 2) to fall from the upper left- hand corner of the paper diagonally ; and the same rule is followed in the more finished drawings where both shade and shadow are introduced. As a means of avoiding the indefi- niteness presented by mere outline, recourse is frequently had to the shading of the parts of a machine or edifice, which is usually done with color and a brush. In architectural drawings, a complete picture is often attempt- ed with all the appliances of shade and shadow, intended to show the artistic effect of the con- struction. Color is introduced in both me- chanical and architectural drawings, to show the material of which the construction is com- posed ; and it is usual to imitate somewhat the natural color of the substances. Besides orthographic projec- tion, architects, for the representation both of exterior and interior of edifices, often make use of perspective; and me- chanical draughts- men, for the better understanding of the parts of a machine than by separate plans and elevations, unite them by the rules of isometrical drawing. The sci- ence of perspective is the representation by geometrical rules, on a plane surface, of objects as they ap- pear 'to the eye from an assumed point of view. All the points of the surface of a body are visible by means of luminous rays proceeding from these points to the eye, forming a cone of rays. The intersection of these rays by an intervening transparent plane is the perspective projection of these points, the rules for the projection of which mechani- FlG.