[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.
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[p. 97] 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.
