COMPASSES.
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.
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.”
Fig. 185.—Fig. 186.—Fig. 187.