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A difference engine is an automatic mechanical calculator designed to tabulate polynomial functions. The name derives from the method of divided differences, a way to interpolate or tabulate functions by using a small set of polynomial coefficients. Most mathematical functions commonly used by engineers, scientists and navigators, including logarithmic and trigonometric functions, can be approximated by polynomials, so a difference engine can compute many useful tables of numbers. The historical difficulty in producing error-free tables by teams of mathematicians and human "computers" spurred Charles Babbage's desire to build a mechanism to automate the process. It is considered to be the world's first computer. == History == (詳細はJ. H. Müller, an engineer in the Hessian army, conceived of the idea of a difference machine. This was described in a book published in 1786, but Müller was unable to obtain funding to progress with the idea.〔Johann Helfrich von Müller, ''Beschreibung seiner neu erfundenen Rechenmachine, nach ihrer Gestalt, ihrem Gebrauch und Nutzen'' (of his newly invented calculating machine, according to its form, its use and benefit ) (Frankfurt and Mainz, Germany: Varrentrapp Sohn & Wenner, 1786); pages 48-50. The following Web site (in German) contains detailed photos of Müller's calculator as well as a transcription of Müller's booklet, ''Beschreibung …'': http://www.fbi.fh-darmstadt.de/fileadmin/vmi/darmstadt/objekte/rechenmaschinen/mueller/index.htm . An animated simulation of Müller's machine in operation is available on this Web site (in German): http://www.fbi.fh-darmstadt.de/fileadmin/vmi/darmstadt/objekte/rechenmaschinen/mueller/simulation/index.htm .〕〔Michael Lindgren (Craig G. McKay, trans.), ''Glory and Failure: The Difference Engines of Johann Müller, Charles Babbage, and Georg and Edvard Scheutz'' (Cambridge, Massachusetts: MIT Press, 1990), (pages 64 ff ).〕 On 14 June 1822, Charles Babbage proposed the use of such a machine in a paper to the Royal Astronomical Society, entitled "Note on the application of machinery to the computation of astronomical and mathematical tables". This machine used the decimal number system and was powered by cranking a handle. The British government was interested, since producing tables was time-consuming and expensive and they hoped the difference engine would make the task more economical. In 1823, the British government gave Babbage £1700 to start work on the project. Although Babbage's design was technically feasible, no one had built a mechanical device to such exacting standards before, so the engine proved to be much more expensive than anticipated. By the time the government killed the project in 1842, they had given Babbage over £17,000, without receiving a working engine. What Babbage did not, or was unwilling to, recognize was that the government was interested in economically produced tables, not the engine itself.〔 The other issue that undermined the government’s confidence in the difference engine was Babbage had moved on to an analytical engine. By developing something better, Babbage had rendered the difference engine useless in the eyes of the government.〔 Babbage went on to design his much more general analytical engine, but later produced an improved "Difference Engine No. 2" design, between 1847 and 1849. Babbage was able to take advantage of ideas developed for the analytical engine to make the new difference engine calculate more quickly while using fewer parts. Inspired by Babbage's difference engine plans, Per Georg Scheutz built several difference engines from 1855 onwards, one of which was sold to the British government in 1859. Martin Wiberg improved Scheutz's construction but used his device only for producing and publishing printed logarithmic tables.〔Raymond Clare Archibald: ''Martin Wiberg, his Table and Difference Engine'', Mathematical Tables and Other Aids to Computation, 1947(2:20) 371–374. ((online review) ) (PDF; 561 kB).〕 During the 1980s, Allan G. Bromley, an associate professor at the University of Sydney, Australia, studied Babbage's original drawings for the Difference and Analytical Engines at the Science Museum library in London.〔(''IEEE Annals of the History of Computing'', 22(4), October–December 2000 ).〕 This work led the Science Museum to construct a working difference engine No. 2 from 1989 to 1991, under Doron Swade, the then Curator of Computing. This was to celebrate the 200th anniversary of Babbage's birth in 2001. In 2000, the printer which Babbage originally designed for the difference engine was also completed. The conversion of the original design drawings into drawings suitable for engineering manufacturers' use revealed some minor errors in Babbage's design (possibly introduced as a protection in case the plans were stolen),〔(Babbage printer finally runs, BBC news quoting Reg Crick ) Accessed May 17, 2012〕 which had to be corrected. Once completed, both the engine and its printer worked flawlessly, and still do. The difference engine and printer were constructed to tolerances achievable with 19th-century technology, resolving a long-standing debate as to whether Babbage's design would actually have worked. (One of the reasons formerly advanced for the non-completion of Babbage's engines had been that engineering methods were insufficiently developed in the Victorian era.) The printer's primary purpose is to produce stereotype plates for use in printing presses, which it does by pressing type into soft plaster to create a flong. Babbage intended that the Engine's results be conveyed directly to mass printing, having recognized that errors in previous tables were not the result of human calculating mistakes but from error in the manual typesetting process.〔 The printer's paper output is mainly a means of checking the Engine's performance. ''Note that, as of November 2015, Difference Engine n° 2 is not currently visible at the Science Museum, as it is being moved to a new exhibit, to open mid 2016.'' In addition to funding the construction of the output mechanism for the Science Museum's Difference Engine No. 2, Nathan Myhrvold commissioned the construction of a second complete Difference Engine No. 2, which is currently on exhibit at the Computer History Museum in Mountain View, California.〔(【引用サイトリンク】title=At the Museum )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Difference engine」の詳細全文を読む スポンサード リンク
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