HarmonographA Victorian parlour machine: a pen on one swinging pendulum, the paper on another. As both slowly wind down they trace a looping figure that never quite closes on itself. The whole shape is decided by the ratio of the two swing rates.Blackburn built the two-pendulum version in 1844. The curve is just two fading sine waves, one for each axis — the same maths as a musical chord, which is why a near-2:3 ratio gives a clean three-lobed figure and an off ratio smears it into a rosette that drifts as the pendulums fall out of step.