The Difference Engine was built in the 90s.

Why has nobody succeeded in building the Analytical Engine?

Charles Babbage designed the Analytical Engine as a general-purpose computer to succeed his more limited difference engines. The Analytical Engine would be Turing complete and programmable in an assembler-like language (conditional branches and loops). Unfortunately, like the designs of his Difference Engine(s), it was not build in Charles Babbage's lifetime.

I am interested in reason for this.

Is the design incomplete? If so, what is missing?

The mill (CPU) itself should be impressive enough (as shown in the Wikipedia article)!

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    One of the fun aspects of the difference engine is that there were not even standardized screws at the time. So, Babbage had to specify the length, diameter & pitch of every little fastener, not to mention all the more real parts...
    – Erik Eidt
    Apr 13, 2018 at 23:10
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    @ErikEidt And now those don't correspond to the modern standardised screws, so they still have to be custom screws; yet another barrier to entry for the Analytical Engine.
    – wizzwizz4
    Apr 14, 2018 at 9:05
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    @PeterParker The issue is that you would then have to redesign the rest of it to handle modern standards. You can't just stick a too-small or too-large screw in a hole not designed for it.
    – JAB
    Apr 14, 2018 at 23:06
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    @Evorlor It was a general purpose fully mechanical computer designed in 1837. It was to be the successor of the Difference Engine, which was a mechanical computer that was limited to mathematics (it was a specialized calculator). Probably the most retro of retrocomputing there is.
    – forest
    Apr 15, 2018 at 6:55
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    Funding was, of course, one of Babbage's problems too.
    – dave
    May 14, 2019 at 11:04

3 Answers 3


Plan 28 is an ongoing effort to do this.

However, it is hard. Nobody is actually sure how to build it.

Winter 2015:

There has been significant archive activity. The major historical source is the Babbage technical archive held by the Science Museum. The Science Museum digitised the archive in 2012 is now preparing to provide open access to the archive. The Analytical Engine project team has been the main user of the digitised archive under special licence. In the course of the project mismatches have been identified between the digitised material and the existing printed index compiled by the late Allan Bromley and published by the Science Museum in 1991. Referencing anomalies, identification of material omitted from digitisation exercise and other structural issues have become evident. Descriptions of these have been compiled and we are working with Science Museum archivists to resolve and correct these ahead of open access release. The work is detailed and, given the volume of material, substantial. Eye-strain is an ongoing hazard.

Spring 2016:

We have been pecking away at Babbage’s original design drawings for some while now and have found with regret that we are unable to reverse engineer a coherent and consistent understanding of the Analytical Engine from the mechanical drawings alone. There are some 300 drawings and some 2200 Notations – descriptions of the mechanisms using Babbage’s language of signs and symbols. There were three phases of design - early, middle, and late.There is overlap between these, there are ad hoc upgrades, and only fragmentary explanation, where there is explanation at all.

Autumn 2016:

Tim Robinson in the US has been trawling through the entire technical archive and compiling a searchable cross-referenced data base for all surviving technical material. In parallel with this I am conducting a fast-track survey of some twenty manuscript volumes of Babbage’s notebooks focussing on material on his notational language (the Mechanical Notation) that he used to describe his machines – this with a view to reading the notational description of the AE designs using the decoded Notation as an interpretative tool to achieve a deeper understanding of the designs. The data-base and trawling exercise is what has primarily occupied us over the last four months.

March 2018:

The Cambridge manuscript belongs to Babbage’s later period (1850s and early 1860s) when he returned to refine and develop his earlier work. The most substantial single section consists of some 65 manuscript pages the transcription of which is now complete. Tim Robinson has vetted the transcriptions and incorporated them into the database. Preliminary review of this new material suggests that while cryptic in parts it is more coherent than previously thought and contains some potentially dramatic simplifications of implementation. This material will be the focus of close study in due course.

Autumn 2018:

The end is in sight creating the cross-referenced database for the set of some 20 Scribbling Books, the manuscript notebooks in which Babbage recorded his workings and thoughts on his engine designs. Tim Robinson, who has been compiling the database is up to the last year of Babbage’s life (1871) and is within striking distance of completion. The process has taken coming up for three years.

March 2019:

The Notations for Difference Engine 1, dating from 1834, thought to exist, had never come to light. These have now been found and represent a crucial piece in the puzzle of the developmental trajectory of the symbolic language Babbage developed as a design aid, to describe and specify his engine, and used extensively in the development of the Analytical Engine.

Equally significant is the discovery of what is thought to be the legendary Plan 28a, part of the most advanced design for the Analytical Engine. There have been references to Plan 28 and Plan 28a designs peppered through the late manuscripts and some design drawings, but the existence of this plan has never been confirmed.

Autumn 2019

Logging this last cache of material is now complete and it appears that only about a third of the original material survives.

With the archive review essentially complete, a process that took over three years, Tim has shifted attention to developing a simulation environment to describe, explore, and verify the mechanical designs. So far this involves ‘logical’ simulation which features aspects of Babbage’s Mechanical Notation, the language of signs and symbols he devised to describe the machines and as a design aid, not unlike a later Hardware Description Language (HDL).

January 2021

With the first-pass inspection of the manuscript archive complete, attention has turned to analysis and interpretation, and organising the findings to aid navigation. Babbage shed versions of the design as it developed in the form of ‘Plans’ – large ‘systems drawings’ which serve as developmental staging posts – the main ones of which number Plan 1 through to Plan 28. The overall approach to analysing the accumulated data is that of a timeline that groups all material, from wherever in the archive (drawings, Notebooks, Notations), to each of the landmark Plans.

Progress was slowed in by climate crises in California not to mention political and pandemic disruption. The initial findings are a long-awaited reward after some four years examining primary sources.

Spring 2021

Excavating further the hardly-known Plan 30 (there is a Plan 28a but seemingly no Plan 29) proved irresistible both for inherent interest and for completeness. Babbage restarted work on the AE designs in June 1857 after a break of almost a decade and referred to the machine as ‘Analytical Engine 30’. Tim reports that the hardware changes introduced for Plan 30 are ‘dramatic’. One remarkable feature is the extension of the Store to 1000 registers, and most intriguingly various methods of mechanically addressing the store contents. The broad-stroke writing has been paused temporarily while this rich seam is explored. It is not expected to take long and we look forward do the resumption of the interpretative account.

Winter 2021:

The project has reached a long-awaited defining point. Tim Robinson has completed the first draft of the most comprehensive description yet of the Analytical Engine designs. We have for the first time both an aerial view that integrates partial and seemingly unrelated developments, as well as the most detailed analysis yet of the specifics of implementation.

This analysis has been a prerequisite for the build. Babbage left no design for a complete Engine and the rationale for the ad hoc improvements made over thirty-eight years has not, till now, been fully investigated nor understood. We have lacked the necessary understanding to inform a meaningful build i.e. which signature features of which design should be combined to create a single representative machine.

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    TL;DR no one is even sure the engine would've actually worked if built according to Babbage's plans Apr 13, 2018 at 20:54
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    @supercat well, we do know that Babbage's ideas were correct and numerous computers were created in the middle of the 20th century based on the same principles. However the real question is whether or not Babbage's design was actually functional. But of course no one is even sure what his design actually was, so its hard to answer that question. Apr 16, 2018 at 21:08
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    @supercat Any changes would likely be related to engineering issues, e.g. a design that is inadequate because it could get stuck. I believe there isn't any doubt that the functional design would work.
    – forest
    Dec 23, 2018 at 9:06
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    @forest: Producing a mechanical calculator which will work if operated sufficiently slowly is not difficult, but making a machine operate smoothly at high speeds without jamming or--even worse--producing erroneous results, is difficult. Given the number of assemblies to be produced, finding the most practical reliable mechanism would have been desirable before constructing too much.
    – supercat
    Dec 24, 2018 at 6:33
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    Until one reached at least 1980s performance levels, doubling the speed of a computer would make it vastly more useful. I don't know that Babbage intended his machine to run any particular number of instructions per second, but he would not have been very happy if, after building a large chunk of it, he discovered that it could only run half as fast as it could have if the design of thousands of parts had each been slightly different.
    – supercat
    Dec 27, 2018 at 20:58

It has been made already (as an emulator)

Thanks to https://www.fourmilab.ch/babbage/

The following is a quote describing their emulator.

Since we're fortunate enough to live in a world where Babbage's dream has been belatedly realized, albeit in silicon rather than brass, we can not only read about The Analytical Engine but experience it for ourselves. These pages include an emulator for The Analytical Engine and a variety of programs for it. You can run the emulator within a Web page or as a command-line application on your own computer (assuming it is equipped with a Java runtime environment). These pages are a museum, and its lobby is the Table of Contents, to which all other documents are linked. Rather than forcing you to follow a linear path through the various resources here, you can explore in any order you wish, returning to the Table of Contents to select the next document that strikes you as interesting. Every page has a link to the Table of Contents at the bottom, so it's easy to get back when you've finished reading a document or decided to put it aside and explore elsewhere.

Here you can find the table of contents.

Here you can run the actual emulator.

I'm not knowledgeable enough to say how accurate this emulator is but I'd appreciate if someone from our fine community would comment below and explain how accurate this emulator is and I'll reflect that in my answer.

So go try it out! Enjoy computing in the 1800s! :)


The Difference Engine was built in duplicate; there are good YouTube videos of it. However, the Analytical Engine was a much-more ambitious project, and building any hardware for it must still be in the future. Its layout reminds one of the Mandelbrot set at 1x scale, so to speak. See also the mechanical computer made, built by Konrad Zuse, and rebuilt, after World War II.

  • 2
    A few sources and citations to back your answer up would be great.
    – Polygnome
    Apr 13, 2018 at 21:56
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    This isn't really an answer to the question - it was "much-more ambitious" isn't an explanation of why nobody attempts to build it. And what does "the Mandelbrot set at 1x scale" mean?
    – wizzwizz4
    Apr 14, 2018 at 16:50
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    Isn't part of what makes the Mandelbrot set so remarkable that it's the same level of complexity regardless of scale? Apr 14, 2018 at 19:30
  • 2
    Nicholas Bodley: The story the Computer History Museum in Mountain View tells is different. The two Difference Engines (serial numbers 1 and 2) were not built at the same time. The first one was built, and when Nathan Myhrvold asked them to make another one for him they didn't want to, but when he generously funded them to make the optional printer they wanted to build, they made him one, and two printers (one for each). When they finished he was moving house and so the museum held onto it for a while (May 2008 to January 2016), where I saw it run. It was amazing! Apr 14, 2018 at 22:33
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    My understanding was that Babbage didn't finish the Difference Engine because he was already thinking of the Analytical Engine; that is, he invented the proud programmer tradition of never finishing a project because you're one-plussing it all the time.
    – dave
    May 14, 2019 at 11:09

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