From a recently declassified history (PDF) detailing the NSA’s computing equipment up to 1964, comes a description of their house-sized computer ABNER’s mercury-powered memory banks:
A succession of pulses (signal or no-signal) travels through an acoustic medium, say mercury, from one end to the other of a “delay line.” […] At the input end of the line is a crystal that converts an electrical pulse to a mechanical wave which travels through the mercury to the other end, where another crystal reconverts it to an electrical signal. The series of electrical signals is recirculated back to input, after passing through detector, amplifier, and driver circuits to restore the shape and strength of the pulses. Also, in the part of the cycle external to the delay line are input and output circuits and “clock” pulses for synchronization. In mercury, the pulses travel at the speed of sound, which is much slower than the speed of electrical signals, and thus the delay in going from one end of the line to the other constitutes a form of storage. […] In ABNER, the mercury tank was a glass tube about two feet long; the delay time was 384 microseconds, or eight words of 48 bits at one-megacycle-per-second rate. Thus the 1,024 words were contained in two cabinets holding 64 mercury delay lines each.
ABNER was named after comic strip character Li’l Abner, reportedly because it was a big, hulking machine that “didn’t know anything”.
(Via Bruce Schneier)