The Difference Engine was a special purpose
calculating machine designed by Charles Babbage to produce tables of
mathematical functions using the method of difference.
The difference engine above is a simple model capable of calculating to only two
orders of difference. It is calculating the tables shown and has reached the
point indicated by the arrows.
The engine consists of three registers linked
together so that additions may be performed from one to the next. The original
three numbers are set in each register by hand. The first turn of the handle
adds the figure in the left hand column to the figure in the middle column; the
second adds the the result of this sum (now shown in the middle column) to the
figure in the right hand column. The cycle starts again and is repeated as
required. The exploded section shows how the columns act upon one another.
The diagram number of wheels (left) shows where the figures are stored. The
Engine has just added the 2 in the right hand column to the 9 in the middle
column. The result 11, appears in the middle column. The next calculation will
add this figure to the 25 in the left hand column. These are indicated by the
arrows in the table.