HOME Charles Babbage  - The Difference Engine and the Analytical Engine
   Home                                            by B. Diane Blackwood
Published in Biographical Encyclopedia of Mathematicians, Donald R. Franceschetti, Editor.  Marshall Cavendish Corporation: New York.  1998.  Pages: 33-36.

The Difference Engine and the Analytical Engine
 Precursors to the modern computer, both the Difference Engine and the Analytical Engine designed by Charles Babbage were calculating machines.  After noticing the common errors in logarithm tables and in everyday mathematical transactions, Babbage conceived the idea of these machines as a means to make precise mathematical calculations mechanically. At first, the British government supported his efforts to eliminate costly computational errors.  It was hoped these machines would correct errors in navigational charts and prevent accounting mistakes.
 The Difference Engine was a machine made out of cogs and wheels.  When a person turned the crank, it would compute simple mechanical calculations.  The major problem with the Difference Engine was that all the pieces had to be in certain positions to start with or the answer would not be correct and the pieces had to be moved back to the starting position each time a new calculation was made.
 Babbage was never able to complete his Difference Engine because he ran out of money and financing at the same time he became more interested in building the more advanced Analytical Engine.  A Difference Engine, using parts that would have been available to Babbage was finally built in 1991 by the National Museum of Science and Technology in London using Babbage's plans. Weighing hundreds of pounds and operated by a hand crank, the Difference Engine has never generated an incorrect answer.
 
 The Analytical Engine was a non-electronic machine with an input device (punched cards), a memorydevice that Babbage called the store, a central processing unit called the mill, and an output device (a mechanical printer), designed to process all types of mathematical equations.  The Analytical Engine used decimal math rather than binary math like modern computers and preformed its computations using repeated addition or subtraction.  It was designed to preform complex mathematical computations and provisions were made for early results to modify later calculations.
 The Analytical Engine was never built because not only did the British government did not want to fund a new engine until the old one was complete but there were significant problems with the mechanical sophistication of Babbage's day.  He had to invent the tools he needed to build his engine.  Conventional mechanical drawing also proved inadequate to his needs causing Babbage to invent his own abstract notation.  By 1851 Babbage had "given up all expectation of constructing the Analytic Engine."

Bibliography
Babbage's Calculating Engines: A Collection of Papers.  Henry  Prevost Babbage.  Los Angeles: Tomash, 1982. Charles Babbage  Institute Reprint Series, vol. 2.
Charles Babbage and his Calculating Engines.  Doron Swade.  London:  Science Museum, 1991.
History of Computing: a Biographical Portrait of the Visionaries  Who Shaped the Destiny of the Computer Industry.  Marguerite  Zientara.  Framingham MA: CW Communications, Inc., 1981.