ISOGONAL CURVES. 



67 



R can slide through a guide which can turn around the pivot P. This pivot is fixed 

 at the point of convergence of the meridians of the chart. At its other end the rule 

 carries a toothed wheel W, which may be fixed in a position, where the edge of the 

 teeth (*. e., of the axis of the wheel) forms any given angle with the meridians. 

 This angle is measured at the dial D. If the wheel is colored and carried along the 

 isogonal curves, it will mark lines of the required direction across them. 



During the motion the wheel partly slides and partly rolls. As the resistance 

 against these two motions is not equal, it requires some care to follow precisely the 



Fig. 62. Machine for tracing line-elements across isogonal curves. 



given curves. It will for this reason be advantageous to have an adjustable friction 

 at the pivots of the toothed wheel. Fig. 63 shows another instrument by which 

 this difficulty is avoided. Instead of a toothed wheel, the rule R carries a drum D 

 with a caoutchouc membrane. This membrane carries a metal plate with a chisel C, 

 which writes a line-element when it touches the paper. By an alternating air- 

 current the membrane is set in motion, making the edge go up and down. When 

 the chisel has this motion and is guided along the curve, it will mark the required 



Fig. 63. Other machine for tracing line-elements across isogonal curves. 



line-elements across it. The desired angle with the meridian can be obtained by 

 turning the drum, which on its upper face carries a dial with the required divisions. 

 The alternating air-current for driving the membrane is obtained from another 

 drum, joined with the crank of a rotating wheel, which is driven by a little 

 electromotor. 



