The bedford canal experiment doesn't prove flath earth
It does not take account of atmospheric refraction. In fact experiments have been carried out on the same stretch of canal that prove the Earth is a globe.
The difference between the two experiments was the height at which the sight line was made. When close to the water there is line of sight along all six miles. When thirteen feet (4m) above the water, the curvature was evident and a pole at the halfway point was obviously higher than the line of sight.
It is common for the atmospheric refraction to exactly cancel the curvature of the Earth, especially over water where there may be expected to be a temperature inversion. Other such experiments have “proved” that the Earth is concave. Of course it is not necessary to have the two effects exactly cancel in order to prove the simple presence of a line-of-sight.
See: Bedford Level experiment - Wikipedia
That link also discounts the popular rule of thumb of 8 inches of dip per mile. It gives it as 8 inches the first mile, a total 32 after the second and 72 after the third.
