in workshop drawings, line-shading is rarely employed, yet
where a design rather than the particular details of construction is to
be shown, line-shading is a valuable accessory. Figure 295, for example,
is intended to show an arrangement of idle pulleys to guide belts from
one pulley to another; the principle being that so long as the belt
passes to a pulley moving in line with the line of rotation of the
pulley, the belt will run correctly, although it may leave the pulley at
considerable angle. When a belt envelops two pulleys that are at a right
angle to each other, two guide pulleys are needed in order that the belt
may, in passing to each pulley, move in the same plane as the pulley
rotates in, and the belt is in this case given what is termed a quarter
twist.

It will be observed that by the line-shading even the twist of the belt
is much more clearly shown than it would be if left unshaded.

An excellent example of shading is given in Figure 296, which is
extracted from the _American Machinist_, and represents a cutting tool
for a planing machine. The figure is from a wood engraving, but the
effect may be produced by lines, the black parts being considered as
simply broad black lines.

[Illustration: Fig. 295.]

The drawings from which engravings are made are drawn to conform to the
process by which the engraving is to be produced. Drawings that are
shaded by plain lines may be engraved by three methods. First, the
drawing may be photo-engraved, in which process the drawing is
photographed on the metal, and every line appears in the engraving
precisely as it appears in the drawing.

[Illustration: Fig. 296.]

For this kind of engraving the drawing may be made of any convenient
size that is larger than the size of engraving to be produced, the
reduction of size being produced in the photographing process. Drawings
for photo-engraving require to have the lines jet black, and it is to
be remembered that if red centrelines are marked on the drawing, they
will be produced as ordinary black lines in the engraving.

The shading on a drawing to be photo-engraved must be produced by lines,
and not by tints, for tints, whether of black or of colors, will not
photo-engrave properly.

It is generally preferred to make the drawing for a photo-engraving
larger than the engraving that is to be made from it, a good proportion
being to make the drawing twice the length the engraving is to be. This
serves to reduce the