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 for Difference Engine
From 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

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