# Primary sources

The Analytical Engine was designed but not built. This makes the source discipline harder than for the Difference Engine — we are reading design documents and reconstructions, not measuring physical artefacts. Provenance tags ("documented" vs "inferred") matter more here than anywhere else in Mechanikon.

## Babbage's design documents

- **Babbage Papers, Science Museum Library, London** — ~7000 pages of drawings, notation, and notebooks accumulated 1834–1871. The "Plans" series documents 30 successive design iterations of the Analytical Engine. Plan 28 is the most complete; the Plan 28 project (below) takes its name from it.
- **Plan 28 project archive** ([plan28.org](https://plan28.org)) — John Graham-Cumming and Doron Swade are cataloguing the Babbage Papers digitally. The intent is to build the engine; the catalogue work is the prerequisite. The project's blog and technical reports are the best modern entry point into Babbage's design state.
- **Charles Babbage.** *Passages from the Life of a Philosopher* (1864). Babbage's own narrative.

## Lovelace's commentary

- **Ada Lovelace.** Notes on L. F. Menabrea's *Sketch of the Analytical Engine* (1843). Public domain. Note G contains the algorithm for computing Bernoulli numbers — the first published algorithm intended to be executed by a machine. Notes A through F discuss the engine's general capabilities.
- The "weaves algebraic patterns" line is from Note A.

## Modern analyses

- **Allan Bromley.** "The Babbage Papers in the Science Museum Library" (1991). The catalogue that opened the design documents to modern scholarship.
- **Doron Swade.** *The Difference Engine* (2001). Discusses the Difference Engine build and the larger Babbage project.
- **John Graham-Cumming.** Various Plan 28 essays and talks on the technical challenges of building the Analytical Engine to Babbage's designs.
- **Tim Robinson.** *The Meccano Analytical Engine* (in progress) — a personal project building a working analytical engine from Meccano parts. Useful as a kinematic reference.

## The mechanism in outline

Components in Babbage's Plan 28 design:

- **Store** — vertical columns, each holding one 40-digit variable. Babbage's design had 1,000 columns in the Store; we render only the first 8 visibly.
- **Mill** — performs the four arithmetic operations on operands transferred from Store. Has an "ingress axis" (input) and "egress axis" (output).
- **Operation cards** — a chain of punched cards, one per machine instruction, fed by a Jacquard-style mechanism. Specifies which arithmetic operation to perform.
- **Variable cards** — a separate chain, specifying which Store columns the Mill should read and write.
- **Number cards** — initial values loaded into Store.
- **Printer** — typeset output of computed results.
- **Stereotype apparatus** — alternate output, stamping a plate for repeated printing.
- **Backing chain** — a feedback mechanism that lets the operation deck advance forward or backward conditionally, providing branches.

## Mathematics that mattered

- **Bernoulli numbers** — Lovelace's Note G computes B₇ = 1/42 via a recurrence involving binomial coefficients. The deck contains 25 operation instructions and uses 7 variable Store columns. Worth running step-by-step in the v0.5 milestone.
- **Bessel functions, logarithms, polynomial roots** — Babbage and Lovelace also discussed these as targets.

## What this sim will not claim

- Exact gear ratios, escapement timings, or carry-mechanism kinematics — Babbage's drawings show several alternative carry schemes, and there is no canonical answer.
- 40-digit precision or 1,000-variable Store — visually impossible at any reasonable scale. We will cap at 5 digits and 8 variables for v0.x; v1.0 may push these.
- That this is "what the Analytical Engine would have looked like." Babbage never finished a complete plan to scale; this is one possible visualisation consistent with the surviving documents.