128 Prof. R. J. Anderson on an Apparatus 



equal. This condition is easily produced by adjusting the 

 weights. 



The octahedron of the second dimetric system, or pyramidal 

 system, is produced by increasing the weights above and below. 



The octahedron of the third system may be easily formed 

 by increasing a pair of the horizontal weights. 



The octahedral figures may be easily formed by leaving out 

 the diagonals and running the cords from the rings at one 

 extremity of the rhombuses through two rings, and then 

 through the opposite ring, to be there fixed to a weight* 

 The tension-weights, as shown in the figure, will then corre- 

 spond to the apices of the rhombuses. 



For the oblique systems further changes are necessary. 

 The upper slide is moved to the right and the lower to the 

 left, or vice versa. This is attended with elongation of the 

 vertical axes, and the cords passing through the pulleys above 

 and below and at the ends are increased, and the slack below 

 is pulled in to a less extent. The other sides of the octahedron 

 are less affected. 



In the first place, the lateral rider-slides are allowed to 

 remain in a position such that the line joining them is per- 

 pendicular to the central vertical longitudinal plane. This 

 gives the Monoclinic System. 



Secondly, the rider- slides are moved one to the right, the 

 other to the left, and in this way the Anorthic or Triclinic 

 System is produced. 



In each case it is desirable to have the slack for each 

 rhombus at different angles of the octahedron. 



All the possible varieties of the fifth system cannot be pro- 

 duced in this way. So it is necessary to arrange for the 

 elevation and depression to the rider- slides in extreme cases. 

 This is accomplished by means of a large ring which carries 

 a pulley. 



I have chosen the octahedron as the simplest figure. 



The cube is formed by the introduction of two horizontal 

 hoops, one above and one below the level of the horizontal 

 bars. These by a simple mechanism are made movable ; and 

 if eight pulleys be fixed opposite the eight edges of the octahe- 

 dron, and the edges of the octahedron be drawn out by rings 

 running on these cords, it will be necessary, then, only to 

 run cords through rings above and below, and to relax* the 

 horizontal and apical weights in order to produce the cube. 



The modifications caused by truncating or bevelling the 

 edges or faces can be produced by increasing the number of 

 the hoops or rings. For the simplest figures, however, vertical 

 hoops answer best. The sliding-rings that are carried by the 



