PANTOGRAPH, an instrument contrived for the purpose of copying drawings, so that the copy may either be of the same, of a greater, or of a less size, than the original. It consists of four moveable rulers fixed together by four pivots, and forming a parallelogram. At the extremity of one of these rulers prolonged is a point, which is drawn over the lines of the original drawing, whilst a pencil, fixed at the end of another branch of the instrument, traces on paper the lines of the copy. The pencil is placed in a hollow cylinder, and a weight is added on the top of the pencil; by this means the point of the pencil is made to press on the paper with the force requisite for drawing the lines. The improvements in the construction of different parts of the pantograph described in the Mémoires de l ’ Acad. des Sciences, 1743, have been generally adopted. The pantograph, however, is not found convenient in practice; and, for the purpose of copying maps, plans, and other designs, artists most commonly employ the method of dividing the original design into a number of squares, and the paper on which the copy is to be made into the same number of squares. When this is done, they copy the lines contained within each square of the original, so as to form a similar figure within the corresponding square of the reticula which covers the copy. When the drawing consists of straight lines, a reduced copy is accurately made by means of an isosceles triangle, in which the base is to the side as any line of the copy is to the corresponding line in the original. By setting off with the compasses any line of the original drawing on the sides of the triangle, the base which completes the triangle is the length of the corresponding line to be laid down on the copy. The same operation is performed by the sector.

If a drawing is to be copied, of the same size as the original, without diminution or enlargement, engravers usually trace the copy upon oiled paper laid over the original, or upon the transparent papier de guimauve (marsh-mallow paper), made in France.

A pantograph has been made, consisting of a rod, placed vertically, and turning round a fixed point in its length, situated between its upper and lower extremity; the up per end of the rod being a point which is conducted by the hand over the lines of the original drawing, and the lower end being the pencil which traces the copy on paper. An instrument upon this principle is sometimes used for drawing the profile of the face, in order to form small portraits or silhouettes.

The instrument called micrograph and prosopograph in Puissant’s Geodesic does not differ in principle from the common pantograph. (b. b.)

A very ingenious instrument for copying drawings, either upon a reduced or an enlarged scale, was invented by Mr Wallace, professor of mathematics in the University of Edinburgh. Of this instrument, which may be fitly introduced under the present head, though differing in name, we have been favoured with the following description.

It is a fact well known, that artists of various descriptions, who have frequent occasion to imitate original designs, have long felt the want of a convenient mathematical instrument, by which a copy may be made with neatness and expedition, that shall have any given proportion to the original. The pantograph is the only instrument that has hitherto been employed; but although correct and plausible in theory, in practice it is found to be so very imperfect that the artist hardly ever thinks of making use of it.

A consideration of the essential service that would be rendered to the graphic art, by a copying instrument, which should be at once simple in its theory and easy in its application, induced Mr Wallace to turn his attention to the subject; and, in the summer of 1821, he produced the model of a copying instrument, which he has denominated an Eidograph (from είδος and γgαfω). The instrument, and its application to the copying of a great variety of subjects, has been shown to engineers, engravers, and other competent judges, in London and in Edinburgh; and their opinion of its utility has been such as to leave no doubt whatever of its completely fulfilling the views of the inventor.
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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∕h∕- crum, 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 maybe communicated from the one to the other. There 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 any 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.