This is employed for drawing elliptic or oval curves, and is represented in Fig. 267. It can be purchased with varying degrees of finish, or may be home made in the following manner: - Two strips of dry hard wood a, 18 in. long, 1 1/2 in. wide, and 3/4 in. thick, are ploughed down the centre to a depth of § in. and a width of 3/4 in.; one is let into the other at right angles so that the bottoms of the grooves or channels are exactly flush, and the structure is strengthened by having a piece of thin sheet brass cut to the shape and screwed down to its upper surface. Next 2 hard-wood blocks 1 1/4 in. long are cut to slide easily but firmly in these grooves, their surfaces coming barely flush with the face of the instrument. A hole is drilled nearly through the centre of each block and about 1/10 in diam., to admit the pins b; and thin strips of brass are then screwed on to the surface of the instrument in such a manner as to secure the blocks from coming out of the grooves while not interfering with the free passage of the pins and blocks along the grooves. To this is added the beam compass c, which consists of a straight mahogany ruler with a narrow slit down the middle permitting it to be adjusted on the pins.

These last may be of brass or steel wire with a shoulder and nut, as at d; they are fixed at the required points on the ruler c, and then inserted in the holes in the blocks, where they are free to revolve. A hollow brass socket e fitted with a pencil is also made to screw on to the beam, and forms the delineator.