FIRST LINES OF A DRAWING.

19

In Fig. 22 draw the lines ab, cd, ef before putting in the lines of the object.

Secondly, with respect to solids; a machine or piece of one. The drawing of any solid object being the projection of that object on a plane or planes, the same statement as made for surfaces applies also for solids.

a

Ө

b

Fig. 23.

Fig. 24.

Figs. 23 and 24 are two views of a cylindrical piece of iron, the line ab is symmetrical with respect to both figures, and is to be drawn first. Other cases will be

noticed as we proceed.

*

These first lines or centre lines should be very fine and distinct; they are not retained in the drawing when inked-in in the case of finished drawings (line, coloured, or shaded); but for working drawings they are, and it is usual to ink them in with red or blue colour, as they are most important lines in such drawings.

* In the examples we shall use a dotted line thus,

instead of a fine line, for the sake of distinction and convenience.

CHAPTER V.

14. A DRAWING of any solid object is the projection of that object on one or more planes, giving one or more views of it. We will state what a projection is, and what are the names given to the different views. In fig. 25 ABCD is the horizontal plane (your drawing

board, for instance); CDEF the vertical plane; let a line ab be held perpendicular to the plane ABCD (and therefore parallel to the plane CDEF) at a distance bc above it and in front of the plane CDEF. If now the line ab is produced to meet the plane ABCD in c, the point c is the projection of the line ab; * from a, b, draw lines perpendicular to the plane CDEF, meeting it in a', b', then a', b' are projections of a, b; and if a', b' be joined, the line a'b' is a projection of ab. c is called a "plan," and a'b' an "elevation" of the line ab. The two planes ABCD, CDEF, which are considered to be at right angles in fig. 25, are really one (the plane CDEF being turned down through a right angle), and are represented by the drawing-paper; the line CD in which the planes intersect is the "ground line" in our drawings, the line which divides the plan from the elevation.

*The point c would be termed a "trace" in practical solid geometry.

PROJECTIONS, PLANS, AND ELEVATIONS.

23

A line perpendicular to the vertical plane, and above the horizontal plane, is shown by d'e', de is the plan of the line. In figs. 26, 27, the planes are shown by two views, fig. 27 being a front-elevation, and fig. 26 an endelevation; the lines ab, de are similarly situated to those in fig. 25. a'b' is an elevation of ab, d'e' an elevation of de. The plan of ab is a point, the elevation of de is also a point.

15. To explain the several terms employed for different views of an object, and to show how these views are obtained, we shall use the object shown in figs. 28 to 31, Plate II. The student is requested to pay particular attention to this part of our subject, and thoroughly to understand it before proceeding to the application of the principles here laid down.

The figure is a prism, whose base is a square upon which it rests, on the horizontal plane (ground); on three faces are placed small prisms differing in length and height, producing an object not symmetrical, as better suited for our purpose. Fig. 28 is a plan, that is, a projection on a horizontal plane. Fig. 29 is an elevation, that is, a projection on a vertical plane situated at AB; the plane is then turned through a right angle, along the line AB (ground line) as an axis, giving the elevation in the position indicated by fig. 29; the arrow shows the direction of projection, the observer being situated at E. If now we wish to have a view of the