when diffused over the surface of the water, show fine
specks, like black dust, on the surface. This is readily apparent,
showing that the mixture of the ink is not homogeneous.

When it is an object to have the lines of a drawing show as black as
possible, as for drawings that are to be photo-engraved, the ink should
be mixed so thickly as to have a tendency to lift when a body, such as a
lead pencil, is lifted out of it. For Patent Office drawings some will
mix it so thickly that under the above test it appears a little stringy.

The thicker the ink can be used the better, because the tendency of the
carbon to separate is less; and it is for this reason that the test
mentioned with a tumbler of water is so accurate. When ink is to be used
on parchment, or glossy tracing-paper, it will flow perfectly if a few
drops of ox-gall be mixed with it; but on soft paper, or on bristol
board, this will cause the ink to spread.

For purposes of measurement, there are special rules or scales of steel
and of paper manufactured. The steel rules are finely and accurately
divided, and some are of triangular form, so that when laid upon the
paper the lines divided will lie close to the paper, and the light will
fall directly on the ruled surface. Triangular rules or scales are
therefore much superior to flat ones. The object of having a paper rule
or scale is, that the paper will expand and contract under varying
degrees of atmospheric moisture, the same as the drawing paper does.

Figure 16 represents a triangular scale, having upon it six different
divisions of the inch. These are made in different patterns, having
either decimal divisions or the vulgar fractions. Being made of steel,
and nickel-plated, they are proof against the moisture of the fingers,
and are not subject to the variation of the wooden scale.

[Illustration: Fig. 16.]

CHAPTER II.

_THE PREPARATION AND USE OF THE INSTRUMENTS._

The points of drawing instruments require to be very accurately prepared
and shaped, to enable them to make clean, clear lines. The object is to
have the points as sharp as they can be made without cutting the paper,
and the curves as even and regular as possible.

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

The lining pen should be formed as in Figure 17, which presents an edge
and a front view of the points. The inside faces should be flat across,
and slightly curved in their lengths, as shown. If this curve is