chapter  1
Difference Engines
Pages 31

Plan 28 is one of the most complex retrocomputing projects ever devised. Begun by John Graham-Cunning in 2010, Plan 28 promises nothing less than the construction of Charles Babbage’s unrealized mechanical computer: the analytical engine. In his TedX Talk given to Imperial College, Graham-Cunning calls the analytical engine “the greatest machine that never was”: a device that processed commands and performed Boolean logic, but was also “as large as a steam locomotive” (TedX). While Babbage had originally proposed his previously designed difference engine in order to help tabulate polynomial functions for use by shipping navigators and scientists, the analytical engine was seen as a more general purpose device. Unfortunately, Babbage’s work on the analytical engine was hampered by a lack of funding and nally halted by his death in 1871. According to Allan Bromley, only an unnished “model of a simple mill and printing mechanism” remained (90). Babbage’s designs for the engine included all of the components of modern-day computers: punch cards designed after Joseph-Marie Janquard’s automatic textile loom; an output mechanism that had a printer, a plotter for graphs, and a Plaster of Paris mold which created stereotype tables; a memory system that held intermediate mathematical results in a set of number axes; and a central processing unit that performed algorithms and made decisions based upon the mechanism of a run-up lever. Eric Roberts explains that the run-up lever “switches from its normal down position into its up position based upon [whether] the sign on the result of the equation is different [than] the sign on the rst number of the equation[,] [or] the resulting answer is greater than 50 digits long.” In each case, the run-up lever would change the behavior of the engine in order to perform decisions and initiate or end looping commands. Graham-Cunning explains that the analytical engine even had a bell, “there were actually instructions on the punch card that read ‘ring the bell.’ […] So, stop for a moment and imagine all those noises. This thing goes ‘click, click, clack,’ steam whistling through its parts, and then DING!”