The Free On-line Dictionary of Computing (30 December 2018):

Difference Engine

    Charles Babbage's design for the first
   automatic mechanical calculator.  The Difference Engine was a
   special purpose device intended for the production of
   mathematical tables.  Babbage started work on the Difference
   Engine in 1823 with funding from the British Government.  Only
   one-seventh of the complete engine, about 2000 parts, was
   built in 1832 by Babbage's engineer, Joseph Clement.  This was
   demonstrated successfully by Babbage and still works
   perfectly.  The engine was never completed and most of the
   12,000 parts manufactured were later melted for scrap.

   It was left to Georg and Edvard Schuetz to construct the first
   working devices to the same design which were successful in
   limited applications.  The Difference Engine No. 2 was finally
   completed in 1991 at the Science Museum, London, UK and is on
   display there.

   The engine used gears to compute cumulative sums in a series
   of registers: r[i] := r[i] + r[i+1].  However, the addition
   had the side effect of zeroing r[i+1].  Babbage overcame
   this by simultaneously copying r[i+1] to a temporary register
   during the addition and then copying it back to r[i+1] at the
   end of each cycle (each turn of a handle).

   Difference Engine at the Science Museum
   (http://nmsi.ac.uk/on-line/treasure/plan/2ndcomp.htm#babbage).

   (1997-09-29)