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all you need to prove the earth is flat is a long straight and calm canal(several exist) and a mile of 2' PVC pipe.
support the pipe with rods that are all the same height off the water.
seal up the tube and suck all the air out.
put a glass window on either side.
look through to hole and see if you can see out the other end.
if the earth is round then all you should see is pipe.