he
idea of a concave surface.

The first thing, therefore, to keep in mind, is this fact: That in all
mechanical drawing, the light is supposed to shine down from the upper
left-hand corner and that, as a result, the lower vertical line, as well
as the extreme right-hand vertical line, casts the shadows, and should,
therefore, be made heavier than the upper horizontal, and the left-hand
vertical lines.

[Illustration: _Fig. 99. Depressed Surface_ _Fig. 100. Concave_]

There are exceptions to this rule, which will be readily understood by
following out the illustrations in the order given below.

PERSPECTIVES.--The utility of the heavy lines will be more apparent when
drawing square, rectangular, or triangular objects.

Let us take Fig. 101, which appears to be the perspective of a cube.
Notice that all lines are of the same thickness. When the sketch was
first brought to me I thought it was a cube; but the explanation which
followed, showed that the man who made the sketch had an entirely
different meaning.

He had intended to convey to my mind the idea of three pieces, A, B, C,
of metal, of equal size, joined together so as to form a triangularly
shaped pocket as shown in Fig. 101. The addition of the inner lines,
like D, quickly dispelled the suggestion of the cube.

[Illustration: _Fig. 101. Fig. 102. Fig. 103. Fig. 104.
Forms of Cubical Outlines_]

"But," he remarked, "I want to use the thinnest metal, like sheets of
tin; and you show them thick by adding the inner lines."

Such being the case, if we did not want to show thickness as its
structural form, we had to do it by making the lines themselves and the
shading give that structural idea. This was done by using the single
lines, as in Fig. 103, and by a slight shading of the pieces A, B, C.

[Illustration: _Fig. 105. Fig. 106. Shading Edges_]

THE MOST PRONOUNCED LINES.--If it had been a cube, or a solid block, the
corners nearest the eye would have been most pronounced, as in Fig. 104,
and the side next to the observer would have been darkest.

This question of light and shadow is what expresses the surface
formation of every drawing. Simple strokes form outlines of the object,
but their thickness, and the shading, show the character enclosed by the
LINES. DIRECTION OF LIGHT.--Now, as stated, the casting of the shadow
downward from the upper left-hand corner makes the last line over which
it passes the thickest, and in Figs. 105 and 106 they are