Eight marbles, one crank, no job. The walnut steps never move. The brass steps rise and fall together, and every top is tilted, so each turn of the crank walks every marble one step up. On the way down a marble rings the watchman's bell, spins through the funnel, flicks the cat's tail and turns the paddle wheel. Then the stairs take it again, until the cord gives out. Stay for the dark.

Every knock and every music-box note is timed by the machine's own crank.

Which machine with no job should we build next?