The strong electric lamp over their working bench dazzled eyes that had become accustomed to candlelight. It shone down on a rack of tools and an orderly collection of drawing materials. A small precision lathe was established on a side bench which was littered with a heterogeneous collection of pieces of metal. A large padlocked cupboard was built into the right-hand wall and beside it Alleyn saw a good modern safe with a combination lock. Three capacious drawers under the bench also were locked.

‘The crucial drawings and formulas have always been kept in the safe,’ Fabian explained. ‘As you can see, everything is stowed away under lock and key. And pray spare a kind thought for my window shutter, so witheringly dismissed by dear old Douglas.’

It was, Alleyn thought, an extremely workmanlike job. ‘And, I can assure you,’ Fabian added, ‘it has not been fiddled with.’ He sat on the bench and began to talk about their work. ‘It’s a magnetic fuse for anti-aircraft shells,’ he said. ‘The shell is made of non-ferrous metal and contains a magnetically operated fuse which will explode the shell when it approaches an aircraft engine, or other metallic object. It will, we hope, be extremely useful at high altitudes where a direct hit is almost impossible. Originally I got the idea from a magnetic mine which, as of course you know, explodes in the magnetic field surrounding steel ships. Now, even though it contains a good deal of alloy, an aeroplane engine must, of necessity, also contain an appreciable amount of steel and in addition, there’s a magnetic field from ignition coils. Our fuse is very different from the fuse in a magnetic mine but it’s a kind of second cousin in that it’s designed to explode a shell in the aeroplane’s magnetic field. As a matter of fact,’ said Fabian with a glint in his eye, ‘if it comes off, and it will come off, I believe, it’ll be a pretty big show.’

‘Of course it will. Very big indeed. There’s one question I’d like to ask. What about “prematures” by attraction to the gun itself, or explosion in transit?’

‘There’s a safety device incorporated in the doings, and it sees to it that the fuse won’t do its stuff until the shell starts to rotate and after it leaves the gun barrel. The result, in effect, is an aerial magnetic mine. I’ll show you the blueprints on presentation of your official card,’ Fabian ended, with a grin.

‘I should be enormously interested. But not tonight, if you don’t mind. I’ve still a job of work to do.’

‘In that case, I’ll escort you back to your room, first making sure that no adventuresses lurk under the benches. Shall we go?’