From: http://www.zetetic.co.uk/zetetic.html -
An important site in the flat earth debate is a long, straight stretch of canal in Cambridgeshire known as 'the Old Bedford Level'. In particular, there is an uninterrupted length of water about six miles long. Over such a distance, according to the globularists, an object placed near the water-line at one end should be rendered invisible from the other by the curvature of the earth. For nine months of 1838 Rowbotham lodged in a wooden hut on the canal bank and conducted a long series of experiments with markers and a telescope to disprove the globularist point of view.
The Old Bedford Level was the scene of further experiments over the years, until in 1904, photography was used to prove that the earth is flat. Lady Blount, a staunch believer in the zetetic faith hired a photographer, Mr Cifton of Dallmeyer's who arrived at the Bedford Level with the firm's latest Photo-Telescopic camera. The apparatus was set up at one end of the clear six-mile length, while at the other end Lady Blount and some scientific gentlemen hung a large, white calico sheet over the Bedford bridge so that the bottom of it was near the water. Mr Clifton, lying down near Welney bridge with his camera lens two feet above the water level, observed by telescope the hanging of the sheet, and found that he could see the whole of it down to the bottom. This surprised him, for he was an orthodox globularist and round-earth theory said that over a distance of six miles the bottom of the sheet should bemore than 20 feet below his line of sight. His photograph showed not only the entire sheet but its reflection in the water below. That was certified in his report to Lady Blount, which concluded: "I should not like to abandon the globular theory off-hand, but, as far as this particular test is concerned, I am prepared to maintain that (unless rays of light will travel in a curved path) these six miles of water present a level surface."