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HOW TO USE THE INSTRUMENTS 27. Drawing Board, and Triangles. Tack a sheet of drawing paper on the board by a thumb-tack at each corner, and let the T square rest on the paper, with the head against the left edge of the drawing board. If the draftsman is left-handed, he should use the T square against the right edge of the board. If now a line be drawn on the paper against the blade of the T-square (Fig. 19), then the T square slid along the edge of the board, and other lines drawn in the same way, these lines will be parallel to one another.

In making drawings, the drawing board should either lie flat on the table or desk, or be raised slightly at the back, giving a forward inclination.

The pencil should always be drawn, not pushed.

Only the upper or working edge of the T square is used in drawing.

28. For drawing vertical or inclined lines, the triangles are used. When one of the shorter sides of a triangle is placed against the edge of the T-square, a vertical line may be drawn against one of the other sides, and a slanting line against the third side. This slanting line will be at 45 degrees for the 45-degree triangle, and at 30 or 60 degrees for the other triangle according to the side of the triangle which is placed against the T-square.

Lines making with the edge of the T-square angles of 15 degrees or 75 degrees, may be drawn by combining the 45-degree and the 30 degree 60-degree triangles, as shown in Fig. 20.

Inclined lines which are not at an even angle, may be drawn parallel to any desired direction as follows: In Fig. 21, AB is an inclined line, and other lines are to be drawn parallel to AB and above it. Place one triangle, as C, with one edge coin ciding with line AB; and place another triangle, as D, against another edge of C as shown. Now, if triangle C slides along triangle D into the dotted positions, lines drawn against the upper edge of C will be parallel to AB; thus BF and GH are parallel to AB.

29. To Draw Lines Perpendicular to a Given Inclined Line. In Fig. 22, AB is the given in

clined line. Place a triangle, as C, with its long est edge coinciding with the line AB; and place another triangle, as D, in contact with the lower left-hand edge, as shown. Next revolve the tri angle C around, as shown by the arrows, into the position EFG; then the long edge in the posi tion EF is perpendicular to the line AB. Other lines perpendicular to AB may be drawn by holding triangle D fast, keeping EG against the edge of D, and sliding triangle EFG along the edge, when lines drawn against the different positions of EF will all be perpendicular to AB.

30. In connection with the T-square, it will be well for the beginner to remember the fol lowing Don'ts: Don't change T-squares while making a drawing. If this must unavoidably be done at any time, see first if lines can be drawn with the second T-square which shall exactly coincide with the first. If not, unless the square has an adjustable head, the paper must be taken off the board, and then readjusted to agree with the second T-square.

Don't use the T-square for a thumb-tack hammer.

Don't use the working edge of the T-square as a straight-edge for trimming drawings.

31. In laying out the border line for a draw ing, if the trimmed sheet is to be only a little smaller than the size of the paper, the center should first be found by means of the diagonals, and the border line laid from the center. If, however, there is ample paper, the border line may be drawn at once without finding the center.

32. Pencils. For line drawing, the pencils should be sharpened to a chisel edge, with cor ners rounded. It is an excellent idea to have the pencil most used sharpened at both ends, one with a chisel edge, and the other with a fine conical point. The latter point is used for making letters, numbers, marking points, etc. The compass lead may be sharpened to a chisel edge or conical point, although here the chisel edge is more satisfactory.