The question is: If you hang two plumb bobs, say 50 feet apart are the
strings parallel?

Theoretically, no, but I doubt if you could measure the difference if
they were just 50 feet apart. If the 2 bobs were, say, a couple
thousand miles apart, they'd both be pointing towards the center of
the earth, so they definitely wouldn't be parallel.
Isn't that the principle that led to the first calculations of the
earth's diameter? Some guy looking down wells... OK, google tells me
it was Eratosthenes, in Alexandria.
Andy