ABSTRACT
The simple metaphor of my title represents a feature of Ultra intelligence which deserves more attention than it has yet received. We still tend to speak of Ultra as if it were an abstract, uniform concept of unvarying consistency, instead of a bundle of awkward practical realities which varied both in quality and quantity of content and also over time. To think of it as a smooth unchanging abstraction is dangerous, for it can mislead interpretation and distort a historical judgement of Ultra's strategic worth. Those of us who helped to make Ultra what it became grew familiar through experience with the sometimes capricious and unpredictable variations in the material from which Ultra was compiled, and I doubt whether anyone who did not serve in Hut 3 at Bletchley Park could gain an equal familiarity now simply by studying the documents in the Public Record Office. Therein lies a certain risk that Ultra may in future be seen as a constant ingredient in intelligence rather than as the living and evolving creature which it really was.
