63) “In a second  experiment Dr. Rowbotham affixed flags 5 feet high along the shoreline, one at every mile marker. Then using his telescope mounted at 5 feet just behind the first flag looked over the tops of all 6 flags which lined up in a perfectly straight line. If the Earth were a ball 25,000 miles in circumference the flags should have progressively dipped down after the first establishing line of sight, the second would have descended 8 inches, 32 inches for the third, 6 feet for the fourth, 10 feet 8 inches for the fifth, and 16 feet 8 inches for the sixth.”

We have no independent source of evidence that his experiment was done, let alone that it produced this result. The only source is Rowbotham’s unsupported claim. It is another case of Rowbotham, who as far as I can tell isn't a qualified Dr., yelling that he did an experiment and found what he was looking for.

Whenever such an experiment of Rowbotham's was tested with impartial sceptical observers involved it was found that the earth curved. 

No valid experiment of today, with the necessary impartial controls implemented, has been able to verify his findings.