(εἰδος and γραφη), a very ingenious instrument for copying drawings, either upon a reduced or an enlarged scale, invented in 1821 by Mr Wallace, professor of mathematics in the University of Edinburgh.

The instrument is represented in the annexed figure.

The beam, AB, which is made of mahogany, slides backwards and forwards in a socket, C; and the socket turns on a vertical axis, supported by the fulcrum, D, which stands on a table. There is a slit in the beam, through which the axis of the socket passes, so that, when the beam slides in the socket, a portion of it passes on each side of the axis. There are two equal wheels, E, E, below the beam, which turn on axes that pass through pipes fixed at A, B, near its extremities; and a steel chain passes over the wheels as a band, by which a motion of rotation may be communicated from the one to the other. There EINE are two arms, F, F, which slide in sockets along the lower face of the wheels, just under their centres; at the extremity, G, of one arm, there is a metal tracer, with a handle attached to it, by which its point may be carried over the lines in any design; and at H, the extremity of the other arm, there is a black-lead pencil fixed in a metal tube, which is ground to fit exactly into a pipe, so as just to slide up or down. In using the instrument, the pencil, in its tube, is raised by a thread which passes over a pulley, and it descends again by a weight with which it is loaded.

From the perfect equality of the wheels, it is easy to see that, if the arms attached to them be placed parallel in any one position, they will retain their parallelism, although one of the wheels, and consequently both, be turned on their centres. Supposing, now, that BC and AC, the parts into which the axis is divided at the centre, have any proportion whatever to each other, if the distances of the tracing point G, and pencil point H, from the centres of their wheels, have the very same proportion, then it follows, from the elements of geometry, that the tracing point G, the centre C, and the pencil point H, will be in a straight line; and further, that CG and CH, the distance of these points from the centre, will have to each other the constant proportion of CB to CA, or of EG to AH. Such being the geometrical property of the eidograph, if the subject to be copied be fixed to the table on which the instrument stands, and the tracing point be carried over every line of the design, the pencil point will trace a copy in all respects similar to the original. To facilitate the adjustment of the instrument, so that the copy may have any given ratio to the original, there are scales of equal parts on the beam and the two arms; by these and verniers, both halves of the beam, and equal lengths on the arms, are each divided into 1000 equal parts, and at certain intervals corresponding numbers are marked on them. By means of the scales, when any ratio is assigned, the adjustment is made without the least difficulty. To avoid any derangement by the chain slipping on the wheels, there are clamps at K and K, which hold it fast to the wheels at points where it never quits them. They are slackened when the instrument is adjusted. (See also Pantograph, and Platometer.)