The second 8,000 piece Difference Engine No.2 is off to California from the Science Museum. It is 3.3m long, 2m high, weighs five tonnes and can calculate polynomials to 31 decimal places.
The second copy of Babbage's Difference Engine No. 2 will be winging its way to California this week as the London Science Museum delivers it to benefactor and former Microsoft CTO Nathan Myhrvold.
"Nathan privately sponsored the printing apparatus for the Museum's machine and funded a complete second engine for his living room in Seattle," former head of Difference Engine construction at the Science Museum Dr Doron Swade told Electronics Weekly.
The Science Museum completed the calculator for its own Difference Engine in 2002 following 17 years of work, but without sponsorship could not afford to construct the Babbage-designed printer to finish the machine.
Through a chance meeting at the Museum, Myhrvold agreed to pay for the construction of the printer if he could buy a complete second machine for himself.
Myhrvold's printer parts were made at the same time as the Museum's, and since then the Museum has been making the second calculator to complete his Engine.
It is not unusual for the Science Museum to make items for elsewhere. "It has to promote the public understanding of science," said Swade, "and originally there were plans to build three Difference Engines, one for the UK, one for the US and one for Japan, but we could not arrange funding."
It also has a commercial arm that manufactures exhibits for other museums worldwide.
Swade has now realised at least part of his original dream, as Myhrvold's Engine will be exhibited and demonstrated for one year at the Computer History Museum, in Silicon Valley
The Science Museum's machine is not called No.2 because it is the second machine constructed after Babbage's failed attempt in the 19th Century, but rather it is what Babbage called that particular design.
In the 1820s Babbage attempted to construct his first Difference Engine, but failed as the design was too complex to be constructed with available Government funds.
"The story that components could not be made precisely enough is not true," said Swade. "We measured original components and Babbage's machinists could make components to any degree of precision required to 1.5 thousandths of an inch [38µm]."
Abandoning this Difference Engine, Babbage went on to design the far more ambitious, and equally unsuccessful Analytical Engine (see below).
Picture courtesy of Doron Swade
However, he developed mechanisms for the Analytical Engine that allowed him to design a far simpler Difference Engine - No.2 - that he never built but the Science Museum eventually did.
"There were no tricks when we built Difference Engine No2. We built it from Babbage's plans," said Swade. "We did have to make some changes: not in the logic, but there were some ambiguities in the drawings and some details he had left for the makers to sort out."
According to Swade, both modern Difference Engines were made on CNC machines to a British Standard tolerance spread identical to that achieved by the original artisans.
The 8,000 parts for each machine (4,000 for the calculator and 4,000 for the printer), were also made using metallurgical copies of the bronze, cast iron and steel used by Babbage.
"When our machine was first built, the handle was quite stiff to turn. One person could do it with two hands," said Swade. "Now it is run-in, you can turn it with one hand. The new one is as stiff as the original was when it was new."
Could you run in-the second No.2 with an electric motor?
"No, it is designed to jam if the result is compromised in any way as a form of error detection," replied Swade.
As well as error detection, the machine is pipelined.
It is designed to add seven 31digit binary numbers, stored on vertical drums, together on to an eighth drum.
Rather than add drum one to two, then two to three, and so on - "only a 25 per cent use of the machine", said Swade, all the odds are simultaneously added to the evens on one half cycle, then all the evens are added to the odds on the other half cycle. See Wikipedia for a more detailed description.
Results are embossed in table form into soft material to be turned into printing pates. Two different sized tables can be embossed simultaneously with the same data, and digit and line spacing are separately adjustable for both tables.
14 pairs of cams control the drums, one cam of each pair pushing the drum one way, and the other cam pushing the drum the other way.
Modification of the cams could allow some degree of microprogramming, said Swade. Springs balanced the mass of the drums and biased them to neutral rest positions.
Babbage's Analytical Engine was a far more ambitions machine that the Difference Engine.
Whereas the Difference Engine printed polynomial tables by simply adding using the 'method of differences', the 50,000 component Analytical Engine would have been able to add, subtract, multiply and divide, and was to be programmed by punched cards and have a large memory.
"It had iterative loops, a von Neumann architecture, micro-programming, pipelining, latching and polling, and conditional branching," said Swade.