﻿llie Duplex Har monograph. 491 



If one receives the traces of an ordinary harmonograph on a 

 table at the top o£ an elliptic pendulum mounted on gimbals, 

 after Benham, the elliptic vibration can be combined with 

 the motion of the other pendulums. By the rise of an elec- 

 trolytic method of tracing, we have already shown how the 

 sense and speed of the motion of the tracing-point can be 

 recorded on har monograms*. 



Harmonographs in general have the defect that the 

 pictures, although entrancingly beautiful, require much skill 

 on the part of the experimenter, combined with good fortune, 

 for their production. In particular, the amplitudes and 

 phases of the constituent vibrations have usually depended 

 on the dexterity and luck of the operator. In the instrument 

 which we shall now describe little is left upon which the ex- 

 perimenter can exercise his skill. When suitably adjusted the 

 instrument will draw the same diagram with the regularity 

 of a printing machine. The amplitudes and. phases of the 

 motion are under control, and the interference of the ex- 

 perimenter consists merely in placing the writing point on 

 the prepared surface and removing it when the record has 

 been traced. 



Tis ley's harmonograph draws the resultant of two simple 

 harmonic motions at right angles to each other. The Duplex 

 Harmonograph replaces each of these motions by the sum of 

 two simple harmonic motions in the same straight line. 



CONSTRUCTION OF THE APPARATUS. 



In fig. 1, A, B, C, and D are knife-edges of four pendulums 

 placed at the corners of a square drawn on the horizontal 

 surface of a thick slate slab. A and C swing in the vertical 

 plane through AG and B and D in the vertical plane through 

 BD. Each pendulum consists of a steel rod 113 cm. long 

 and 2'2 cm. in diameter. Knife-edges are placed 22 cm. 

 from the top and swing in grooves cut in flat steel rings 

 placed over holes in the slate slab at the corners of the 

 56 cm. square. A centimetre of needle is fixed vertically to 

 the top of each pendulum and passes through a hole in a 

 light wooden rod. The rods on A and B meet at right 

 angles at c in the centre of the square, where they are 

 pierced by a sewing-needle (forming a hinge) with its point 

 downwards. The point of this needle draws the curves on a 



* ' Knowledge/ Jan. 191: 



