The head at a valve that was shut in no time at all

The pressure head at the downstream end of a 1,200 metre line after the valve is closed instantaneously, in metres of water. It steps up by the Joukowsky rise of 204 metres, holds for the 2.4 seconds the wave takes to reach the reservoir and come back, then inverts — and the train decays only because the pipe has friction in it.

4 essays call how-long-a-machine-remembers. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about.

It is also the blast radius. Changing how-long-a-machine-remembers changes every figure listed here at once, and on this site it may change what they assert as well as what they show — which is what has to be rebuilt and looked at before the change is believed.