dimensions, for stepping off series of equal dimensions, and - -as their name suggests- -for dividing dimensions, etc., into various numbers of parts by trial.
For advanced work a variety termed " hair spring " is to be preferred. These have the point of one leg attached to a spring controlled by a small milled head screw. The coarse adjustment is made in the usual way by pressure of the finger on the legs, then very minute final adjustment is made by turning the screw-head.
Bow Comprises, Fig. 16.--These are smaller and lighter than the full-size instruments. They are made in sets of three, having fixed pen, pencil and divider points, and are of two sizes, 3 in. and 3J in. long. These are very convenient for describing smaller circles and curves.
Bow-Springs, Fig. 17, are miniature compasses also made in sets of three. The legs are formed of spring steel and are normally open to their greatest radius about -J- in., but may be closed by turning the milled head screw to describe circles down to in. radius. They are usually sold in sets :'n velvet- lined cases, but single bows are also supplied. They are useful for drawing very small circles and curves.
The Beam Compasses.—These are pen and pencil legs, inserted into brass sockets, which are then adjusted upon a lath or rod, called the " beam." They are used for describing circles of greater radius than the compasses will extend. Fig. 18 shows a home-made arrangement in mahogany that will be readily understood upon inspection
The Ruling; Pen, or. as it is often miscalled, drawing pen, Fig. 19, for it is quite unsuitable for drawing in the usual sense of the term, consists of a pair of adjustable steel nibs fixed to a straight handle. The better sorts have one of the nibs hinged to enable it to be more thoroughly cleaned, and to permit the removal of the burr formed on the edges when resharpening. Its use will be described fully later, and it need_ only be said here that it is used with a straight or curved ruler for inking-in pencilled lines.
The Parallel Rule, Fig. 20, is an instrument for
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