Compasses consist of two pointed legs; they are instruments for
describing circles or for--sometimes--measuring figures, in absence of
dividers. Fig. 175 represents compasses fitted as dividers.
Compasses should have jointed legs, which will allow the points to be
placed at right angles to the paper, whatever the size of the circle to
be drawn. Compasses should not be used for circles which are too large
to allow the points to be thus placed; a lengthening bar is generally
provided, which greatly increases the diameters of circles which may be
drawn by this attachment; it is shown in fig. 176.
One leg of the compasses is usually provided with a socket to which are
fitted three points: a divider point, fig. 179; a pencil point, fig.
177; and a point, fig. 178, carrying a special pen for the inking of
circles. Each of these points is generally provided with a joint, so
that it may be placed at right angles to the paper.
The other leg should be jointed; it is often provided with a socket
which receives two points, one a divider point, and the other carrying a
needle point. Such an instrument may be used as dividers for spacing, or
as compasses for penciling or inking circles.
The joint at the head of the compasses (see fig. 175) is the most
important feature. It should hold the legs firmly in any position, so
that in going over a circle several times only one line will result. It
should allow the legs to move smoothly and evenly, and should be capable
of adjustment.
[Illustration: Fig. 180.--Fig. 181.--Fig. 182.--Fig. 183.--Fig. 184.]
As shown in fig. 174, one leg has a hinge or joint, and a needle point,
which can be regulated by a thumb screw; the other leg has a socket or
recess into which interchangeable parts can be inserted. The four
figures to the right of the compasses show the parts which are provided
with shanks or insertion pieces. Fig. 180 and fig. 181 represent
compasses specially used for making small circles, and work too minute
for the larger instruments described above.
To do work of this nature easily a pair of spring dividers are
frequently used. This instrument has one point attached to a spring,
which is regulated by a screw, so that very slight changes in the space
may be made with ease.
Compasses specially used for putting in fine circles and dimensions are
called “bows.” When a pen point it is a “bow pen,” with a pencil point a
“bow pencil,” and if with needle point a “bow dividers.” Fig. 180 is a
“bow dividers”, this fitted with screw for fine adjustment in one leg,
fig. 181, is called a “hair-spring bow dividers”; for small details,
bows with steel spring legs without any joint are used; these are called
“steel-spring bows.”
[Illustration: Fig. 185.--Fig. 186.--Fig. 187.]