Page:On the economy of machinery and manufactures - Babbage - 1846.djvu/141

Rh An instrument was contrived, a long time ago, and is described in the Manuel de Tourneur, by which copper-plate engravings are produced from medals and other objects in relief. The medal and the copper are fixed on two sliding plates at right angles to each other, so connected that, when the plate on which the medal is fixed is raised vertically by a screw, the slide holding the copper-plate is advanced by an equal quantity in the horizontal direction. The medal is fixed on the vertical slide with its face towards the copper-plate, and a little above it.

A bar, terminating at one end in a tracing-point, and at the other in a short arm, at right angles to the bar, and holding a diamond-point, is placed horizontally above the copper; so that the tracing-point shall touch the medal to which the bar is perpendicular, and the diamond-point shall touch the copper-plate to which the arm is perpendicular.

Under this arrangement, the bar being supposed to move parallel to itself, and consequently to the copper, if the tracing-point pass over a flat part of the medal, the diamond-point will draw a straight line of equal length upon the copper; but, if the tracing-point pass over any projecting part of the medal, the deviation from the straight line by the diamond-point, will be exactly equal to the elevation of the corresponding point of the medal above the rest of the surface. Thus, by the transit of this tracing-point over any line upon the medal, the diamond will draw upon the copper a section of the medal through that line.

A screw is attached to the apparatus, so that if the medal be raised a very small quantity by the screw,