In 1985 the Science Museum in London set out to construct a working Difference Engine No. 2 built faithfully to Babbage's original designs dating from 1847-9. The project was led by the then Curator of Computing, Doron Swade. The purpose of the project was both to memorialize Babbage's work in time for 200th anniversary, in 1991, of Babbage's birth, and at the same time to resolve two nagging questions: could Babbage have built his engine, and had he done so, would it have worked?
Babbage left twenty large design drawings which depict the Engine's mechanisms. Detailed as they are, they are insufficiently detailed to serve as manufacturing drawings. Tolerances, methods of manufacture, choice of materials and finish are not specified. New drawings were made specifying each of the 8,000 parts with sufficient detail for manufacture. Modern manufacturing methods were used but uncompromising care was taken to ensure that the precision achievable by Babbage was nowhere exceeded. Composition analysis on materials used by Babbage was carried out to ensure the best materials match.
The project took seventeen years to complete. The calculating section was finished in 1991 in time for the bicentenary of Babbage's birth, and the printing and stereotyping apparatus was completed in 2002. The project had a drama worthy of Babbage - funding crises, manufacturing challenges, impossible deadlines, and technical puzzles.
The completed machine works as Babbage intended. Its 8,000 parts are equally split between the calculating section and the output apparatus. It weighs five tons and measures seven feet high, eleven feet long and is eighteen inches deep at its narrowest. As a static object it is a sight to behold - a sumptuous piece of engineering sculpture. In operation it is an arresting spectacle.
The project confirms Babbage's standing as a designer of formidable ingenuity. It also demonstrates that achievable precision was not a limiting consideration in Babbage's failures. It appears that the 19th century outcome had as much to do with politics, economics, and personalities, as with technology. We can say with some confidence that had Babbage built his engine, it would have worked.
The complete working Babbage engine is on public display at the Science Museum in London. A duplicate engine and printer, a 'second original', was recently completed for a private benefactor of the project, Nathan Myhrvold, formerly chief technology officer and Group VP at Microsoft. Myhrvold has generously agreed to delay delivery of the Engine to him and lend it to the Computer History Museum, Mountain View, California, where it will be displayed and demonstrated.