LINES IN DRAWING.--In drawing, lines have a definite meaning. A plain circular line, like Fig. 95, when drawn in that way, conveys three meanings: It may represent a rim, or a bent piece of wire; it may illustrate a disk; or, it may convey the idea of a ball.
Suppose we develop them to express the three forms accurately. Fig. 96, by merely adding an interior line, shows that it is a rim. There can be no further doubt about that expression.
Fig. 97 shows a single line, but it will now be noticed that the line is thickened at the lower right-hand side, and from this you can readily infer that it is a disk.
SHADING.--Fig. 98, by having a few shaded lines on the right and lower side, makes it have the appearance of a globe or a convex surface.
[Illustration: _Fig. 96. Ring_ _Fig. 97. Raised Surface_ _Fig. 98. Sphere_]
Shading or thickening the lines also gives another expression to the same circular line.
In Fig. 99, if the upper and left-hand side of the circle is heavily shaded, it shows that the area within the circle is depressed, instead of being raised.
DIRECTION OF SHADE.--On the other hand, if the shading lines, as in Fig. 100, are at the upper left-hand side, then the mind at once grasps the 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.