IIR mercury-based memories part of a larger class of memory called "delay-line memory" that dates all the way back the 1920s. The basic premise is that it uses the time delay of signals propagating in some medium to temporarily "store" a value.

The modern analog is the need to refresh DRAM in order to keep the bits flipped the right way.

https://en.wikipedia.org/wiki/Delay_line_memory