資料來源 : Free On-Line Dictionary of Computing

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://www.nmsi.ac.uk/on-line/treasure/plan/2ndcomp.htm#babbage)}.
     
        (1997-09-29)