est
condition.

A circle-pen is shown in Figure 24, in which A represents the point-leg
and B the pen-leg. The point-leg must be the longest because it requires
to enter the drawing paper before the pen meets the surface. The point
should be sharp and round, for any edges or angles on it will cause it
to widen the hole in the paper when it is rotated. To shape the points
to prevent the enlargement of the centre in the paper is one of the most
important considerations in the use of this instrument, especially when
several circles require to be drawn from the same centre. To accomplish
this end the inside of the point-leg should be, as near as possible,
parallel to the length of the instrument (which is denoted in Figure 24
by the dotted line) when the legs are closed, as in the figure. If the
point is at an angle, as shown in Figure 25, it is obvious that rotating
it will enlarge the top of the centre in the drawing paper. The point
should be sharp and smooth on its circumferential surface, and so much
longer than the pen-point that it will have sufficient hold in the paper
when the instrument stands vertical and the pen-point meets the surface
of it, which amount is about 1/64th of an inch.

We may now consider the shape of the pen-point. Its inside surfaces
should be flat across and to the curve shown in Figure 24, not as shown
exaggerated in Figure 25, because in the latter the body of the ink will
be too near the pen-point, and but little can be placed in it without
causing it sometimes to flow over the edges and down the outside of the
pen.

A form of pen-point recently introduced is shaped as in Figure 26, the
object being to have a thin stream of ink near the marking pen-point and
the main body of the ink near at hand, instead of extending up the pen,
as would be the case with Figure 24. The advantage thus gained is that
the ink lies in a more solid body, and having less area of surface
exposed to the air will not dry so quickly in the pen; but this is more
than offset by the liability of the ink to flow over the crook at A, and
cause the pen to draw a thick ragged line. The pen-point must be
slightly inclined toward the needle-point, to the end that they may
approach each other close enough for drawing very small circles, but it
should also stand as nearly vertical as will permit that end to be
attained. As this pen is for drawing small circles only, it does not
require much ink, and hence may be somewhat clos