Twelve pendulums hang from one brass bar, each a little longer than the one before, so each swings a little slower. While the shortest swings 22 times, the longest swings 11, and every one in between swings once less than its neighbour. So the row drifts out of step into snakes, twos and threes, and every 25.6 seconds it falls back into one straight line. A comb of spring teeth gives each one a tiny push every swing, and the conductor beats time on the same rod. But something on this machine is worn, and it is not saying what.

Every knock and every music-box note is timed by the machine itself.

Stay for the end.

Which machine with no job should we build next?