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Comment by randomwalker | Hacker News Reader

That's possible, but an alternative explanation for the cost overruns that I've read is that Babbage had terrible project management skills.

Wikipedia has this to say:

In 1991, the London Science Museum built a complete and working specimen of Babbage's Difference Engine No. 2, a design that incorporated refinements Babbage discovered during the development of the Analytical Engine. This machine was built using materials and engineering tolerances that would have been available to Babbage, quelling the suggestion that Babbage's designs could not have been produced using the manufacturing technology of his time.

https://en.wikipedia.org/wiki/Analytical_Engine

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I just finished reading The Difference Engine yesterday, an entire book about this project, by the project lead!

https://www.amazon.com/Difference-Engine-Charles-Babbage-Com...

(unfortunately overshadowed in Google Search results by a William Gibson book of the same name)

It gives a lot of color on Babbage, but yes the conclusion was that Babbage design basically worked, and could have been built. There were errors in his drawings that they had to correct, but nothing fundamental.

The group at the Science Museum spent over 6 years doing this! This is the group that holds most of his papers, drafts, and unfinished machines.

Although there are a couple things I want to follow up on. They weren't that specific about what computation they did. And does it still work today? It was extraordinarily finicky. It produced a lot of bit errors, as did mechanical computing devices that came later, which sort of defeated the purpose (it was supposed to calculate tables of logarithms and such with higher accuracy than humans.)

They require tolerances that could be achieved in the day, but not reliably was certainly not standard practice. When the funding was cut they were basically running a research program to develop better machining. With the research program failed to bad project management is kind of irrelevant. My point was it was still in the area of being a research program.

The examples built do indeed keep on working with non-prohibitive maintenance - the 2nd #2-design engine (built in the 2000s for Nathan Myhrvold), was on display at the CHM in Mountain View for 8 years with daily or twice-daily demonstration runs. It sadly went off display in 2016 (probably to go to Myhrvold's private collection) but I saw the demonstration a couple of times and can answer some of your questions:

1. The concrete computation performed was to use the Finite Difference Method (https://en.wikipedia.org/wiki/Finite_difference_method - hence the "Difference Engine" name) to calculate arbitrary polynomials of degree up to IIRC 10. By using Taylor Series, this method could be used to calculate arbitrary functions, like log and sine. This was in fact the same method used to construct logarithmic tables by hand at the time, and had similar nominal precision; the singular goal was to eliminate the bit errors rampant in the old, manual process.

2. The machine removed not just errors in calculation, but also in typesetting; about half the part-count of the original design was in its printer, which could be configured with all kinds of options for typesetting the results. It would output a "print preview" onto paper locally (this was not publicly demonstrated at the CHM because of the enormous mess of ink spills, but the machinery was run dry), and an identical wax mold ready for use in mass printing. This was because many of the bit errors in the existing log/sine/etc. tables were introduced not by the (human) computers, but by the multiple copying steps involved in transforming calculated values into printed pages.

3. Computation was quite reliable - the machine worked in base 10, and mechanisms were carefully designed to freeze up (and be easily resettable to a known-good state, as demonstrations showed) before introducing errors. As far as I know bit errors were unheard of in the demonstration runs. This reliability, like in later electronic computation, was the motivation for using digital rather than analog logic. (Finickiness was mostly limited to those halting conditions - it proved quite sensitive to clock speed (rate of crank turn), but only by the standards of the hand cranking used in demonstrations; connected up to a steam engine with 19th-century rate governors, input power could have been kept clean enough to run with long MTTF.)

Thanks for those details. I am glad to have seen the demonstration. I didn't know they removed it in 2016.

I believe the better link is https://en.wikipedia.org/wiki/Divided_differences

I think an important difference between Babagge’s attempt and that of the London Science Museum is that, for the latter, it was much more economical to produce the parts to the required tolerances.

The difference between “takes an expert engineer a month to create” and “can be CNC’d in a day” is enormous, even if the end results are the same.

They still haven’t built the Analytical Engine, which was the general-purpose computer and significantly more complex.

Also, building something with the benefit of 150 years of technological progress gives you quite a leg up on the past even if you use techniques that would have technically been available back then.

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