In [Fig. 1175] is shown what is known as a pair of hermaphrodite calipers, which consists of two legs pivoted at the upper end; the bent leg is placed against the perimeter of the work, as shown, and held steadily, while with the point a line is marked on the work. This operation is performed from four equidistant (or thereabouts) points on the work, which will appear as shown in [Fig. 1176], providing the radius to which the point was set be equal to the radius of the work. The point at which the lines meet is in this case the location for the centre. If, however, the radius to which the points are set is less than the radius of the work, the lines will appear as in [Fig. 1177], in which case the location is in the centre of the inscribed square, as denoted by the dot; or if the radius be set too great the lines will appear as in [Fig. 1178], and the location for the centre will again be as denoted by the dot.
Fig. 1179.
Fig. 1180.
Another and very old method of marking these lines is to place the work on a pair of parallel pieces and draw the lines across it, as shown in [Fig. 1179], in which w represents the work, p, p the parallel pieces of equal thickness, s a stand (termed a scribing block) carrying a needle n, which is held by a thumb screw and bolt at b. The point of the needle is adjusted for the centre of the work, a line is drawn, the work is then rotated, another line drawn, and so on, until the four lines are drawn as in [Fig. 1180], when the work may be turned end for end if light, or if heavy the scribing block may be moved to the other end of the work.
The centre locations are here made true with the part of the work that rests on the parallel pieces, and this is in some cases an essential element in the centring.