Martin,
I think the only connection between a bilge pump on ship(there were two basic designs, the one I mentioned above and a wheel type affair) and the rocker-arm rail trolleys is a matter of engineering(not my forte:) There must be some efficiency of operation that makes that particular design appropriate for the two different applications. Personally, I'm just glad I don't have to do wither for aliving.
And to ya'll, as to folk music. It is a discussion I find extremely interesting and fun to pursue, and of value for the sake of the conversation itself. I find it meaningless(and even dangerous) when we try and impose "rules" on what constitutes folk music and then judge singers/performers by our criteria.
To paraphrase Dr. Malcolm in "Jurassic Park", "the songs will find a way."
My personal criteria for folk music? - It "sounds" like a traditional folk song - I like it - Let the folk decide
When I look at my repertoire it is certainly a mix of oral tradition, composed songs, old and new. And yet it definitely has a commonality to it, all the songs fit thematically and musically into a boundary that feels traditional Anglo-Irish, and listeners seem to understand those distinctions.
Now let's talk about "The Unicorn":)
Cheers. Marc