Difference Engine
The difference engine was the first machine that is now considered as a computer. The British mathematician Charles Babbage conceived the difference engine in 1822. The difference engine was only partially built when Babbage conceived the idea of another, more sophisticated machine called an Analytical Engine. One of Babbage's co-workers was Ada Lovelace, for whom the Ada programming language was subsequently named.
Babbage intended his difference engine to automatically calculate mathematical tables, such as logarithmic and trigonometric functions, particularly as they applied to navigation tables. Accurate navigation data was of great interest to the British in Babbage's day, for use by their sea-going merchant and naval vessels. The machine was intended to be steam-powered and fully automatic, even to the point of printing out the resulting tables. It was also to be controlled by a fixed instruction program, somewhat analogous to the modern day computer operating system.
The difference engine was envisioned as a replacement for the human beings that performed the laborious mathematical calculations. Due to their function, it was the people who were then referred to as computers. This designation continued for a further century and a quarter, until the term became synonymous with the machines now performing the calculations.
The machine took its name from the method to be used for the calculations—the method of differences. The method of differences utilizes an algorithm to produce tables of numbers generated in series, starting with some arbitrarily low number, with incremental increases thereafter. Each row in the table is tallied, and the tally is squared to generate the tally number in the succeeding row. The difference between these two tallied numbers becomes the number inserted in the central column of each row. The machine is a visual representation of the table, consisting of columns of wheels. Each wheel is serrated, and has ten digits marked along its edge. Between the nine and zero there is a projecting tooth. The teeth enable the linkage of the wheels in each column. If a wheel, in receiving addition, passed from nine to zero, the relevant projecting tooth pushed over a lever, bringing the next wheel above it into play. In effect, the device is a calculator, capable of storing the data generated and reusing this information to produce more data.
In Babbage's lifetime, a functioning difference engine was never constructed. Babbage did produce a prototype in 1822, and began construction of the functional version a year later. Then, this project was set aside, as he became interested in the analytical engine. The difference engine finally did become reality in 1991. At that time, a team at London's Science Museum constructed a cast iron, bronze, and steel version according to Babbage's original hand-drawn plans. The machine stood over 6 feet tall, was 10 feet wide, and weighed three tons. A sequence of calculations was solved correctly to 31 decimal places, more accurate than most modern-day pocket calculators. Because a crank powered the machine, hundreds, sometimes thousands of turns of the crank were required for each calculation. Currently, the printing functioning of the difference engine is being incorporated into the museum's version of the engine.
The recognition of the importance of the difference engine has come relatively recently historically. After its design, the machine faded into obscurity. Within the last several decades, however, its significance to computing was recognized, in the worlds of science, computing, and even literature. With respect to the latter, the machine formed the title and theme of a 1991 book co-authored by science fiction stalwarts William Gibson and Bruce Sterling.
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