What his particular setup was i could not say, but it sounds as though he had constructed a rather charming mercury delay line. The rough principle is that you send off a signal (basically a sound wave) through your medium (in this case mercury) and set up a detector to listen for the echo. if there was an echo, you repeated the signal and kept on doing it every few milliseconds as necessary. such a signal could count as a stored binary "one". An absence of signal would be a stored binary zero. If your pathway through the mercury was long enough, you could store a whole string of bits in one column of mercury.
The Ferranti Mercury computers in the 1950s used loops of mercury for their main storage. It was not the only possible delay line medium, but it did have certain technical advantages.
Go well,
Jon
