l, resting by its own weight on the paper, is guided round by
gently turning, without pressure, the small knob at the upper end of the
tube. By this means the misplacing or sliding of the centre-point and
the cutting of the paper by the pen are avoided. By means of this fixed
centre-point any number of concentric circles may be drawn, without
making a hole of very distinguishable size on the paper.

[Illustration: Fig 37.]

In applying the ink to the bow-pen as to all other instruments, care
must be taken that the ink lies between the points only and not on the
outside, for in the latter case the ink will flow down too freely and
make a broad, ragged line, perhaps getting on the edge of the square
blade or triangle, and causing a blot of ink on the drawing.

In using a straight line or lining pen with a T square it may be used as
in Figure 36, being nearly vertical, as shown, and moved from left to
right as denoted by the arrow, S representing the square blade. But in
using it, or a pencil, with a straight edge or a triangle unsupported by
the square blade, the latter should be steadied by letting the fingers
rest upon it while using the instrument, the operation being shown in
Figure 37. The position, Figure 36, is suitable for long lines, and that
in Figure 37 for small drawings, where the pen requires close adjustment
to the lines.

CHAPTER III.

_LINES AND CURVES._

Although the beginner will find that a study of geometry is not
essential to the production of such elementary examples of mechanical
drawing as are given in this book, yet as more difficult examples are
essayed he will find such a study to be of great advantage and
assistance. Meantime the following explanation of simple geometrical
terms is all that is necessary to an understanding of the examples
given.

The shortest distance between two points is termed the radius; and, in
the case of a circle, means the distance from the centre to the
perimeter measured in a straight line.

[Illustration: Fig. 38.]

[Illustration: Fig. 39.]

[Illustration: Fig. 40.]

Dotted lines, thus, <----->, mean the direction and the points at which a
dimension is taken or marked. Dotted lines, thus,-----, simply connect
the same parts or lines in different views of the object. Thus in
Figure 38 are a side and an end view of a rivet, and the dotted lines
show that the circles on the end view correspond to the circle of the
diameters of the head and of the