In the production of all great constructive works the drawing plays a most important part. If a piece of machinery, a ship, an aqueduct or a temple is to be built, verbal descriptions would be insufficient directions to the workmen who are to perform the actual labor; drawings become a necessity, because a drawing tells exactly what is meant, where words would utterly fail. Therefore, to everybody connected with the constructive trades, to artisans in whatever field, the ability to read, if not to make, a drawing becomes a necessity; and to those in positions of authority the ability to make a drawing is the power to convey their ideas to others. That branch of drawing with which the pattern cutter has to deal is of a purely geometrical nature and is properly termed orthographic projection. The term orthographic (signifying right line) is well applied because it exactly describes the nature of the work, as will be seen further on.
The geometrical drawings made use of in representing any constructive work, whether to a large or a small scale, are of three kinds - viz.: Elevations, sections and plans. The term diagram is sometimes used in connection with this class of drawing, but is not of a specific nature. It means a drawing of the simplest possible character, usually made to demonstrate a principle, and may partake of the properties of either of the above named drawings.
An elevation, if the word were judged by its common meaning, would be understood to show the hight of anything. It does this and more. It gives all the vertical and horizontal measurements which appear in the front, side or end which it represents. An elevation supposes the observer to be opposite to and on a level with all points at the same time, and is therefore an impossible view, according to the rules of pictorial art. Being always drawn to scale (including full size), it gives exact dimensions of hight and breadth at any part of the view, but furnishes no view of horizontal surfaces and no means of measuring distances to and from the observer, or in any oblique horizontal direction. An elevation may be called front, side, end or rear, according to the relative dimensions of the object, one of whose faces it represents. Any elevation or vertical section gives two sets of dimensions - i. e., hight and horizontal distance, which lie parallel to the face which it shows.
A Section, as the word indicates, is a view of a cut or a view of what remains after certain portions have been cut away for the purpose of showing more clearly the interior construction. The idea of a vertical section can best be described by supposing that a wire stretched taut, or any perfectly straight blade, was passed vertically down through an object at a given distance from one of its ends or sides, indicated by a line in some other view or views, and the portion not wanted was removed. The view made of the section may properly include only the parts cut. or if made to include or show portions that would naturally appear by the removal of the parts, it would properly he called a sectional elevation. Sections may also be taken horizontally at any hight above the base or ground line, indicated by a line for that purpose upon one or more of the elevations.
Horizontal sections are properly classed with plans. Vertical sections are known as longitudinal or transverse, according as they are taken through the long way of, or across, an object. Elevations or sections may also be constructed upon oblique planes when necessary to more fully show construction.
Sections of small portions or members drawn to a large scale or full size are called profiles. They are applied to continuous forms, as moldings, jambs, etc., and are drawn for the purpose of showing the peculiarities in form of the parts which they represent.
The view which gives all the horizontal distances in whatever direction is called the plan. The name plan applies equally well to a horizontal section or to a top view. In the plan, as in sections and elevations, the observer is supposed to be opposite to (i.e., directly above) all points at the same time. In idea it is the same as a map, the difference between the two terms being in the amount, included in the view.
In Fig. 128 is given an illustration of the various geometrical views of an object, placed in their proper relation one to another, showing the lines of projection and the lines upon which the different sections are taken. A house placed upon a base has been selected as the most suitable object for purposes of explanation in the present case. It has been shown in diagrammatic form - that is. denuded of all cornices, trimmings or projecting parts - so as to demonstrate the principles of projection in the clearest manner possible.
It rests with the designer to determine which of the views shall be drawn first, all depending upon the given facts or specifications in his possession. If a house is to be designed, it is most likely that the plan would be drawn first, as arrangement of rooms and amount of ground to be covered would be of the first importance. If a molding be the subject of the design, the profile would be the view in which to first adjust the proportion of its parts. The method of deriving the elevation from the section or obtaining any one view from one or more other views is termed orthographic projection, because by it a system of parallel lines is made use of for the purpose of obtaining the same hight (or width, as the case may be) in corresponding parts in the different views,
In this connection it is to be understood that each angle or limit of outline in a sectional view is the source of a right line in the elevation. In Fig. 127 is shown, at X, a sectional view or profile of a molding, which should be so drawn that all the faces or surfaces supposed to be vertical shall lie vertically on the paper; that is, parallel to the sides of the drawing board. To project an elevation, Y, from this section, place the T-square so that the blade lies horizontal - that is, crossing the board from side to side - and bring" it to the various angles A, B, C, etc., of the profile, drawing a line from each. The point E, though not an angle, is the lowest visible point or limit of that member of the mold when seen from the front, and is, therefore, entitled to representation in the elevation by a line. In like manner the point D, being the upper limit of a curve, is entitled to representation, but being so situated as to be invisible when viewed from a point in front of the mold, the line is properly made clotted, The lines of projection from the section to the elevation are also shown dotted in the engraving. A vertical line terminates the elevation of the mold at the right or end nearest the section, while the absence of such a line at its left end indicates that it extends indefinitely in that direction. It would also be proper, upon that supposition, to finish the elevation at the left with a broken line.