Babbage's Difference Engine No. 2
Charles Babbage (design); built by the Science Museum (1791 to 1871) · designed 1847 to 1849; built 1985 to 2002
A wall of brass and steel columns, each stacked with numbered wheels, stretches more than three metres along its frame, with a printing unit at one end and a great crank handle at the other. This is a calculating machine designed by the Victorian mathematician Charles Babbage between 1847 and 1849, but never built in his lifetime. The Science Museum constructed it from his drawings, finishing the calculating section in 1991 for the two hundredth anniversary of his birth and the printer in 2002. It works. Turn the handle and eight thousand parts move together to compute polynomial tables to thirty one digits and stamp the results into a soft mould for printing.
The machine calculates by the method of differences, adding fixed numbers repeatedly so that it never needs to multiply. Each column holds one number as a stack of wheels marked zero to nine, and a carry mechanism ripples up the columns when a wheel passes nine. Look at the sheer regularity of the parts: Babbage insisted that mass produced identical components were the key, and the modern builders proved that Victorian machine shops could have made them to the tolerances required. Nothing more precise than what was available in the 1840s was used.
Babbage spent decades and a fortune of government money on his earlier Difference Engine No. 1, of which only a portion was completed; that demonstration piece is also in the museum. He fell out with his engineer, moved on to the far more ambitious Analytical Engine, and died disappointed. Building No. 2 answered a long argument: the failure had been about money and personalities, not about the design. A memorable detail is the printer, which can typeset results in two different layouts, an idea a century ahead of its time.
Did You Know? Babbage never saw a working engine; when the museum finally built one to his drawings, it computed correctly on the first full run.
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