Work in Progress (February 1995)

This account documents the construction of Difference Engine No. 2 taking the original set of Babbage's drawings as an uninterpreted datum. The purpose of the account is to place on record the detailed interpretation of a Babbage engine design, it also serves to provide a technical record of the process by which a project, arrested in 1849, was resumed and culminated in the completion, in 1991, of an operational machine.

Foreword

The accompanying account describes Charles Babbage's Difference Engine No. 2. The machine was built by the Science Museum to original designs dating from between 1847 and 1849. The engine was completed in 1991, the bicentennial year of Babbage's birth. The set of original design drawings consists of twenty main views and a small number of derivative tracings. The set is believed to be complete i.e. there is no evidence of drawings being missing. The drawings collectively constitute a comprehensive operational description of the engine: they are sufficiently detailed to describe the shape and nominal size of individual parts, their physical interrelationship and their intended function. However, for all the richness they contain the drawings are not 'working drawings' i.e. they are not sufficiently detailed to provide the full specification necessary for the manufacture of parts. No information is provided in the originals as to choice of materials, methods of manufacture, requisite precision, or finish. In this respect they provide a schematic description, comprehensive in itself, but insufficiently detailed to serve directly as a specification for manufacture.

The drawings are deficient in other respects: they contain dimensioning inconsistencies (the same parts are shown differently sized in different views); there are incompletenesses in the design, instances of omitted devices, as well as redundant assemblies. The drawings also contain design errors one of which is central to the fundamental operation of the engine. None of these deficiencies compromises the validity of the basic logic, design or intended function of the engine. Babbage made no practical attempt to construct the engine and the deficiencies for the most part represent the gap between an advanced design arrested in an incomplete stage of engineering development, and the final working mechanical entity. The gap, in short, is one of engineering completeness rather than logical or operational principle. The drawings constitute the only original account of the engine. Babbage provides almost no other explanation, textual description or justification for the design. The drawings are therefore a free-standing source from which his intentions are to be decoded and reconstructed.

The manufacture of parts required fully specified working drawings. These include detailed piece-part drawings, parts lists specifying quantities of individual components, layouts and general assembly drawings detailing the physical interrelation of assembled parts. The production of these drawings required detailed interpretation of the originals to establish the purpose and function of parts and subassemblies. This interpretation then informed the provision of supplementary data, otherwise missing from the originals, but essential for manufacture. Composition analysis of contemporary gunmetal informed the choice and grade of modem bronze; which parts where to be made from bronze, cast iron or steel, methods of manufacture, finish and tolerancing were informed by expert curatorial advice based on detailed knowledge of nineteenth-century practice. The guiding principle throughout was authenticity, and care was taken to ensure that no part was made with greater precision than is known from measurement to have been deliverable in Babbage's time. Dimensioning inconsistencies were resolved, incompletenesses in the design remedied, modifications to correct design errors specified and omissions and redundancies catered for. These researches and deliberations were embodied in a set of some fifty working drawings which completely specify the engine. During the design and build Reg Crick, the senior engineer responsible for the production of the modem drawings and for the supervision of the construction, kept an Engineer's Log (The Red Book) which is an episodic diary of notable issues. Apart from this we were as guilty as Babbage during the design and build phases of the project in providing little descriptive account of the technical issues underpinning the design. The attached account is intended to remedy this, at least in part.

The documentation process started in January 1993 and has continued fairly consistently since then. The account is based on debriefing sessions between myself and Reg Crick. During the debriefing sessions the interpretation of the original drawings is retraced and reexamined so as to reconstruct the supposed intention and function of the various devices. Deficiencies, where they occur, are identified, and the means and justification by which they were resolved described. The Engineer's Log (The Red Book) is used as a checklist of notable issues. My role in these sessions progresses from disciple, to inquisitor, and finally, to that of scribe. The debriefing sessions are recorded in handwritten summary session notes by me. These are then taken away and used in solitude as the basis of an expanded account written usually on the same day or in the days immediately following. In the next session Reg Crick would proof read the text Secondary questions raised during the drafting process, as well as amendments to the text from the proofing process would be discussed, clarified and resolved. The account would then be redrafted accordingly and re-read at the start of the following session. The pages attached are the outcome of the process to date.

There are two flagged incompletenesses in the account. These are provisional omissions which require curatorial research outside Reg Crick's or my immediate knowledge. One of the topics is the use of Babbage's Mechanical Notation. Despite the central importance attached by Babbage to the Notation as an interpretative and descriptive tool, we did not use it to decode the drawings nor in the construal of Babbage's intentions. While I have used the Notation in the later sections as a short-hand to identify parts, this is the main use to which it has so far been put in the written record. The account will need to be reviewed in the light of a more informed understanding of the Notation to verify and confirm interpretative decisions taken without its aid. An investigation of the notational conventions devised by Babbage is a necessary curatorial research task but is being held in abeyance until the technical account, which relies on Reg Crick's unique experience and knowledge, is complete. The second issue is the historical account of a major design error in the addition mechanism. This requires reconstructing the sequence of interpretative exchanges between Prof. Allan Bromley and the engine team. Retracing the trajectory of understanding here is not essential to the debriefing process. This too has been postponed for the meanwhile but will need to be returned to for completeness.

The engine depicted in the original design drawings consists of a calculating section to which is attached a printing and stereotyping apparatus. Only the calculating section has so far been built. The description of the calculating section is retrospective i.e. this part of the engine existed as a fait accomplis at the time of writing. The account is therefore 'descriptive' in the literal sense (de - down, scribo - I write) and my role here is largely that of a scribe retracing, recovering, articulating and recording, via a process of understanding, a set of considerations and events already enacted. The account, however, also covers the printing and stereotyping section which remains unbuilt but for which working drawings were produced in the interval since the bicentennial year, funded by independent sponsorship from the United States. The relationship between the process of documentation and the process of design is in this case different.

The printing and stereotyping section is integral to the concept of the machine and forms an essential part of the engine's control system. The apparatus is at least as complex as the calculating section, and calls for an additional 4,000 parts - about the same number as required for the calculating section already built - but with substantially less repetition of similar parts. Documenting the printing and stereotyping apparatus involved the familiar process of re-examining the existing interpretation of the original design intention, and retracing the decisions made in designing the incompletely designed parts of the mechanism. The opportunity for curatorial scrutiny was greater here than circumstances allowed during the earlier build of the calculating section, and since there still existed the possibility to re-interpret the design, the curatorial responsibility was correspondingly greater. The process of reviewing the design substantially altered our understanding of the printing mechanism and revealed features of startling subtlety formerly overlooked. The design was subsequently altered to accommodate this partial re-interpretation. I cite this to illustrate how, in the case of the printing apparatus, the documentation process and the design process directly interacted and breached what had until then been separate activities.

The account consists of seven sections which collectively run to 45,460 words. A total of 300 hours has been spent on debriefing and writing up since January 1993 (192 hours in 1993, 96 hours in 1994, and 12 hours in 1995). The ratio between debriefing time and write-up is about 1:2 i.e. each hour of debriefing requires a further two hours of formulation, research and writing. The technical account based on one-to-one debriefing sessions is within an estimated 6,000 words of completion (2,000 words outstanding for the stereotype apparatus and 4,000 words for the final section which will describes the assembly of the engine). The writing rate is typically 180-200 words per hour.

The order in which the sections are presented is in the reverse order of writing i.e. the earlier sections are last, the most recent first The genre of writing is highly intemalist and allows little linguistic licence. The earlier sections (appearing last) tend to be costive, taut and formal. As the genre developed the style, though inevitably still constrained, tends nonetheless to be slightly more expansive.

Doron Swade

25 February 1995