Oil painting of Charles Babbage


Babbage designed Difference Engine No.2 between 1847 and 1849. The design is elegantly simple and requires three times fewer parts than Difference Engine No. 1 for the same computing power. The design benefits from development work done on the Analytical Engine which is more complex.

Babbage made no attempt to construct Difference Engine No. 2 and like his other Engine designs none was completed in the metal. The reasons for his failure continue to exercise historians. Factors cited include Babbage’s allegedly difficult personality, unconvincing progress, disputes with his engineer, Joseph Clement, political instability and the eventual withdrawal of government funding, though the view most often repeated in histories of computing is that Babbage’s failure was due to limitations in Victorian machine tool technology.

To explore the thesis that the limitation of Victorian engineering was a contributory factor in Babbage’s failure to complete any of his machines the Science Museum set about constructing Babbage’s Difference Engine No. 2 in 1985. Before the engine could be constructed the original design drawings were redrawn and expanded to provide the engineering detail needed for modern manufacture. In addition a small trial piece was built to verify the original addition mechanism and the mechanism for carrying tens.

The calculating section of Difference Engine No. 2, has 4,000 moving parts (excluding the printing mechanism) and weighs 2.6 tonnes. It is seven feet high, eleven feet long and eighteen inches in depth and built to original designs using materials closely matching those available to Babbage. Modern techniques were used in the manufacture of repeat parts but care was taken to restrict limits of precision to those achievable by Babbage.

Difference Engine No. 2 was the first full sized Babbage calculating engine to be completed. It was made as a research machine for display at the 1991 exhibition commemorating the bicentennial year of Babbage’s birth.

Babbage had conceived of the Analytical Engine by 1834 after the Difference Engine project collapsed. It is the Analytical Engine more than the Difference Engine that shows Babbage’s forward thinking.

The Difference Engines were automatic i.e. they did not rely (as did the manual calculators that came before) on the continuous informed intervention of a human operator to get useful results. The Difference Engines were the first designs to successfully embody mathematical rule in mechanism. However, the Difference Engines are not a general purpose machines. They could process numbers entered into them only by adding them in a particular sequence. The Analytical Engine was not only automatic but also general purpose i.e. it could be `programmed’ by the user to execute a repertoire instructions in any required order.

The engine was envisaged as a universal machine for finding the value of almost any algebraic function. The Analytical Engine is not a single physical machine but a succession of designs that Babbage refined until his death in 1871.