[Q] Nowadays, I suppose aircraft designers do their modelling on digital computers, you were doing this long before they were available, so did you do it with pencil or paper or did you have some kind of analogue models that you used?

Well, no, basically you did it with pencil and paper. You had one... computing aid, the slide rule. I would never move anywhere without a slide rule, I mean, I couldn't imagine how a man could live without a slide rule for a time, you know, everything depended upon it. I don't have one anymore, obviously. They were great machines, quiet and clean and obedient, you know, nice things, actually, slide rules. It did... I don't think it was altogether wasted. I mean it is true that one could spend one or two days doing a single numerical computation, who... on the answer of which might depend somebody's life. And it does concentrate the mind. I am very good, still, at doing numerical computations and not making mistakes. And when computers came in, the experience I'd had as a hand calculator was really helpful in... in setting up computations. But you asked about analogue computers. We did and we didn't, it's curious, this. We didn't have any digital computers around, but one of the things you had to know before an aeroplane flew was you had to know what we called it's Natural Modes of Vibration, how it would shake, how it would vibrate, because you needed to know that in order to calculate what speed it would actually vibrate itself to pieces, the so-called Flutter Speed. And it's very difficult to work out the natural modes of vibration of a complicated structure. So what we used to do, before the aeroplane was built, was to build what was actually an electrical analogue of the mechanical system, in which the masses were replaced by inductances, and the... the stiffnesses were replaced by capacitances in the thing and so on. And we would then shake, electrically, the electrical analogue, and deduce from that how the structure was going to...

[Q] How could you be confident that the analogue that you... the electronic analogue obeyed the same mathematical laws, presumably you knew it did up to a point, but how could you be confident that it would... for all cases?

Well, it's something that... how shall I put it...?

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