COMPASSES. There are many varieties of these useful instruments. The Common Compasses, or Dividers, are simply two pointed legs on a common pivot, for transferring dis tances. For drawing a circle the lower end of one of the legs is removed, and its place sup plied by a bolder for a pencil, or by a steel pen. In the Hair Compasses, one of the legs has a part attached to the upper part by a spring, so that by means of a screw a very small motion may be given to the lower end ; the instrument is convenient for very accurate dividing, but must be used with care. Trian gular Compasses have three legs and two pivots, so that the three points of a triangle can be at once transferred ; the instrument is useful only in rough work, as it is difficult to handle. Proportional Compasses consist of two dividing compasses with a common pivot, which, when open, present vertically opposite angles ; con sequently, the intervals between the points of one and the other are in the same proportion as the legs of one to the legs of the other. The pivot is a clamping screw, which can be transferred along the interval between the pairs of points ; and a scale points out how to adjust the instrument to alter any line, or sur face, or solid, in a given proportion. These
compasses sometimes have an apparatus for slight adjustment. Beam Compasses consist of a cylindrical bar, perpendicular to which, with clamping screws, slide a point and a pen cil. The use of it is to describe large circles, or measure large distances, the common com passes being very liable to slip when opened very wide. It is a very safe and sure construction.
A machine used as a substitute for com passes, in describing a curve passing through three or more points nearly in a straight line is made as follows :—An elastic rod of metal is furnished with a rigid bar on which it can be drawn up by screws, so that the rod shall form an arc, the chord of which is a part of the bar. This may he adjusted so as to pass through the given points when the curve is to be traced along the front of the rod. This is sometimes called a Shipwrights' Bow.