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Third 'given': Protective Field.
If you look at Searle's work (which I haven't, myself, for quite some time) I think you'll find accounts of a powerful 'force field' being generated around the device -- at least on the outer edge...
My personal opinion about flying saucers is that they are shaped that way for purely functional reasons (I'm an artist, sometimes a designer -- not a scientist) and that a single system provides power, propulsion, AND the protective field.
I cannot begin to tell you how this works, though part of me has an intuitive understanding of it.
Best of luck!
- fil
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