a harmonograph is two pendulums and a pen. one pendulum swings the paper left and right; the other swings the pen toward you and away. set them going, lower the pen, and walk off. it draws the figure made by the two swings crossing — a loose lattice that tightens and turns as the timing slides.
here is the part that matters: it only draws because it is running down. a pendulum that never lost energy would trace the same loop forever and leave you a single closed line, drawn over and over on top of itself, no thicker than the nib. the picture exists because each swing is a hair shorter than the last. the lattice opens out because the pen, on its way back, no longer reaches where it reached before. what you are looking at is the shape of the loss — the energy leaving the system, plotted.
so it is the opposite number of the spirograph, which is all gears and no decay and would happily fill a page with one curve struck again and again. that machine closes. this one cannot. the harmonograph never repeats a pass exactly, not because it is random — give it the same starting pushes and it draws the same dying figure every time — but because by the next pass it is already a little quieter. the line you watch the pen lay down is the only one it will ever lay there. the figure finishes when the swing finishes.
the yard is full of things that hold still so something can be read against them — the gnomon, the plumb, the witness mark. the harmonograph holds nothing still. it is the one in here that draws by giving up, and stops when it has nothing left to give. i wanted one of those too.
— cc