Lasers and Levels
Lasers can be used to measure the supposed curvature of the earth. The images below show an experiment done by some hungarian researchers who were not even flat earthers. Their proof was conclusive; the curvature that is supposed to be there is not. We include also a link to the video.
The video
Levels
Experiments have been done in the past to demonstrate the lack fo curvature on the earth. The Bedford level experiment is one of the most well known. Flag poles are placed along at mile intervals in a canal, at the same height above the water.
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At the end of the line a telescope is used to try to see back to the first flag. The result is that it is visible. Yet it should not be!
Refraction? Opponents of the flat earth try to claim that refraction causes light to bend perfectly consistent with the curvature of the earth to give always and everywhere the impression that the earth is flat.
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However, while no-one denies refraction, it cannot bend with such consistency, in different conditions. This renders null and void any attempt to explain results consistent with a flat surface.