Re: flash in the pan
The explanation JMurray found in Phrase Finder is the one I have always seen. I don't think panning for gold was widespread in England in 1687, when Mr Settle wrote: "If Cannons were so well bred in his Metaphor as only to flash in the Pan, I dare lay an even wager that Mr. Dryden durst venture to Sea." Muskets (and real-life flashes in the pan) had been around for some three centuries then.
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