THE HARMONQGRAPH. 



179 



following manner : on the two consecutive sides of a drawing board I fixed 

 four small pieces of wood (fig. 1 80), side by side in twos, having at the end a 

 small piece of tin-plate forming a groove (fig. 181). In these grooves nails 

 are placed which support the pendulums. The piece of wood is placed on 

 the corner of the table, so that the pendulums which oscillate in two planes at 

 right angles, are in two planes that are sensibly parallel to the sides of the 

 table. The pendulums are made of a thin lath, with two small pieces of 

 wood fixed to them containing some very pointed nails, on which the 

 pendulum oscillates. Fig. 182 gives an illustration. The pendulums have a 



Fig. 184. Details of mechanism. 



pin fixed in vertically, which passes through a piece of wood, and by means 

 of a hinge connects the upper ends of the two pendulums. This contrivance 

 of the pin is very useful, and if care is taken to make the hole through the 

 hinge in the form of a double cone, as shown in fig. 182, c, it makes a perfect 

 joint, which allows the piece of wood to be freely moved. 



To complete the apparatus, the heads of the two pendulums are united 

 by the hinge, at the bend of which a slender glass tube is fixed, which traces 

 the curves. The hinge is given in fig. I 84, and to its two extremities are 

 adjusted the two pins of the pendulum (fig. 183). The pendulums are 

 encircled with round pieces of lead, which can be fixed at any height by 

 means of a screw. 



