# Harmonograph

27 26 The laTeral ocTave 21 figures of eight and wings After Unison the next harmony to try is the Octave. Here there is a technical difficulty, for the variable pendulum has to be very short, and because of the greater amount of friction involved runs down quickly. The trick is to add a weight to the top of the invariable pendulum, which slows it down see the picture on the title page. The variable pendulum can then be longer. Unfortunately this means that for the octave, and other ratios where one pendulum is going much faster than the other, the theoretical markers often have to be ignored, and the right point found by trial and error. With one pendulum beating twice as fast and at rightangles to the other, the octave in open phase takes the form of a figureofeight a coincidence, repeated in diminishing size as the pendulum runs down. If both pendulums are released at the same time to produce the closed phase, the result is a cupshaped line that develops into a beautiful winged form with fine crosshatchings and interference patterns. Small adjustments produce striking variations. The octave is the first overtone see page 8.