COMPASSES of Three Legs, or Triangular Compasses, are, setting aside the excess of a leg, of the same structure with the common ones. Their use is to take three points at once, so as to form triangles, and for similar purposes.
Beam-Compasses consist of a long branch, or beam, of brass or wood, carrying two brass cursors, the one fixed at one end, the other sliding along the beam, with a screw to fasten it occasionally. To the cursors may be screwed steel or pencil points. Beam-compasses are used to draw large circles, to take great extents, &c. To the fixed cursor is sometimes applied an adjusting or micrometer screw, by which an extent is obtained to extreme nicety.