What is the difference between a calculator and a computer? There is no one clear-cut right answer to this question, but many historians point to this document as one of the most important papers in the history of computing. Charles Babbage designed two kinds of mechanical computational machines: a “difference engine,” or calculating machine; and an “analytical engine,” which was far more than a mere calculator. In 1840, Babbage presented his design for the “analytical engine” to a group of mathematical engineers in Turin, Italy. One of them, Luigi Menabrea, who would later become Prime Minister of Italy, published an account of Babbage’s design that appeared in Geneva. With Babbage’s encouragement, Augusta Ada King, Countess of Lovelace and the daughter of Lord Byron, translated Menabrea’s article into English and added her own substantive commentary and explanatory notes.
Lovelace’s notes went considerably beyond what Babbage and Menabrea had written. Her lengthy appended notes amount to 40 pages of very dense text compared with only 24 pages, lightly spaced, for Menabrea’s article. Lovelace explained how Babbage’s “analytical engine,” if constructed, would amount to a programmable computer rather than merely a calculator. It would take input from punch cards, and store variables for use in diverse sequential operations. (Examine the column headers on the fold-out plate; some of these are designated for variables.) Babbage’s ingenious design achieved, through purely mechanical operations, a functional equivalence to the conditional branching, looping, and parallel processing operations of early electronic computers. Although Lovelace showed how Babbage’s engine could generate a Bernoulli series of numbers, she argued for the wider potential of the engine to produce analytical results beyond the realm of mathematics.
Lovelace explained: “Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent.”
Babbage concluded, speaking of Menabrea’s article with Lovelace’s notes: “These two memoirs taken together furnish, to those who are capable of understanding the reasoning, a complete demonstration - That the whole of the developments and operations of analysis are now capable of being executed by machinery.”
A translation of Menabrea’s article, originally published in French, appears on pp. 666-690; Lovelace’s notes on pp. 691-731.
The Computer History Museum in San Jose, California, has built a working model of Babbage’s second Difference Engine. Babbage’s Analytical Engine has never yet been constructed.
See the account of Lovelace in James Gleick, The Information: A History, A Theory, A Flood (New York: Vintage Books, 2011); and the biography of Ada Lovelace at MacTutor.