PENTAGRAPH

, otherwise called a Parallelogram, a mathematical instrument for copying designs, prints, plans, &c, in any proportion.

The common Pentagraph (Plate xix, fig. 2) consists of four rulers or bars, of metal or wood, two of them from 15 to 18 inches long, the other two half that length. At the ends, and in the middle, of the long rulers, as also at the ends of the shorter ones, are holes upon the exact fixing of which the perfection of the instrument chiefly depends. Those in the middle of the long rulers are to be at the same distance from those at the end of the long ones, and those of the short ones; so that, when put together, they may always make a parallelogram.

The instrument is fitted together for use, by several little pieces, particularly a little pillar, number 1, having at one end a nut and screw, joining the two long rulers together; and at the other end a small knot for the instrument to slide on. The piece numb. 2 is a rivet with a screw and nut by which each short ruler is fastened to the middle of each long one. The piece numb. 3 is a pillar, one end of which, being hollowed into a screw, has a nut fitted to it; and at the other end is a worm to screw into the table; when the instrument is to be used, it joins the ends of the two short rulers. The piece numb. 4 is a pen, or pencil, or portcrayon, screwed into a little pillar. Lastly, the piece numb. 5 is a brass point, moderately blunt, screwed likewise into a little pillar.

Use of the Pentagraph.—1. To copy a design in the same size or scale as the original. Screw the worm numb. 3 into the table; lay a paper under the pencil numb. 4, and the design under the point numb. 5. This done, conducting the point over the several lines and parts of the design, the pencil will draw or repeat the same on the paper.

2. When the design is to be reduced - ex. gr. to half the scale; the worm must be placed at the end of the long ruler numb. 4, and the paper and pencil in the middle. In this situation conduct the brass point over the several lines of the design, as before; and the pencil at the same time will draw its copy in the proportion required; the pencil here only moving half the lengths that the point moves.

3. On the contrary, when the design is to be enlarged to a double size; the brass point, with the design, must be placed in the middle at numb. 3, the pencil and paper at the end of the long ruler, and the worm at the other end.

4. To reduce or enlarge in other proportions, there are holes drilled at equal distances on each ruler; viz, all along the short ones, and half way of the long ones, for placing the brafs point, pencil, and worm, in a right line in them; i. e. if the piece carrying the point be put in the third hole, the other two pieces must be put each in its third hole; &c.