Bedford Level Experiment

In Norfolk, England, northeast from Welney, is Old Bedford River. It is a slow moving canal that goes on in a straight line for six miles uninterrupted, and it has often been used as an experiment to verify the flatness of the Earth. In addition to being an argument, it has some historical significance to being favored by Samuel Rowbotham, who inspired a number of historical flat earthers from Lady Elizabeth Blount to Samuel Shenton.

The Experiment
Three poles are fixed at equal height above the water along the six mile length of the river. As the surface of the water should follow any curvature of the Earth, so too should the tops of the poles.

However when one observes through a theodolite, one finds that the tops of each pole are level. This could only occur on a Flat Earth. Over six miles, RET predicts 24 feet should be obscured.

Historical Accounts
The first notable account of this experiment was by Samuel Birley Rowbotham in 1838, though he did so only with a telescope watching a boat sail away. He repeated this several times.

The above experiment was first performed in 1901, by Henry Yule Oldman, a reader at King's College, Cambridge.

Another notable instance was in 1904. Lady Elizabeth Blount, founder of the Universal Zetetic Society, arranged for a commercial photographer to take a photo of the sheet placed on the surface of the river, from a low mounted camera just two feet above the water. These images should have been impossible on a Round Earth, and were so published widely.