PANT'AGRAPH (Gr. panto, graphein, to delineate), an instrument by the aid of which any engraving may be copied on paper, though its use is in practice restricted to the copying of maps and plans. The copy-can be drawn to any scale. The instrument consists of rods, AB, AC, DF, and EF, jointed together, as in the figure: the points D and E are so taken that AD is equal to EF, and AE to DF, and consequently ADEF is always'a parallelogram. If C be a determinate point near the end of the rod AE, and any line, CHB, be drawn cutting the other three rods, the triangles BAC and BDH are similar; so that when the point B is fixed, the points C and II, which can, from the structure of the instrument, move in any direction, will describe similar figures differ ent in size; that described by C being to that described by H in the proportion of CB to HB. The practical working of the instrument is as follows: The points H and B are determined by the ratio BH to BC, which is the proportion the scale of the copy bears to that of the original; a socket, which slides along the arm, is fastened exactly at B on the under side; below this is placed a heavy weight, with a stalk fitting into the socket, thus rendering B the center of mo tion of the instrument, if the weight be heavy enough. A pencil is fitted into another socket
at II, and a rod of metal with a sharp point, called the tracer, is fastened at C, and the in strument is fitted with castors at various points underneath, to allow of its being moved freely. The operator then passes the tracer over the outline to be copied, and simultaneously the pencil at II makes the copy on the required scale. If a copy on a scale nearly as large as the original be required, the fulcrum must be 6 placed in DF, and the pencil in DB; while, if a magnified copy be required, the pencil and tracer must exchange the positions assigned them in the first case. The defects of this in strument are its weight and the difficulty of rendering it perfectly mobile, both of prevent that steady motion of the tracer which . 0 is necessary for making an accurate copy.
To remedy these defects, the pantograph has been constructed in a variety of forms, all of which, however, like the one described, depend upon the principle that the two triangles which have for their angular points, the fulcrum the pencil-point and a joint, and the fulcrum the tracer-point and a joint, must always preserve their similarity. '