A barrel organ engraved with the subject of Die Kunst der Fuge. Three voices read the same twelve notes at three speeds, one note per second, per minute and per hour, so the outer voice states the subject five times a minute while the inner one takes half a day to state it once. That is canon by augmentation, and it is also just a clock. On the hour the barrel plays B♭–A–C–B♮, the four notes Bach spelled his name with.