The Bedford Level Experiment is a series of observations carried out along a six-mile (9.7 km) length of the Old Bedford River in the UK during the 19th and early 20th centuries, to measure the curvature of the Earth. Samuel Birley Rowbotham, who conducted the first observations starting in 1838, claimed he had proven the Earth to be flat. However, in 1870, after adjusting Rowbotham's method to avoid the effects of atmospheric refraction Alfred Russel Wallace found a curvature consistent with a spherical Earth

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Advocates of a flat Earth, however, were not deterred: on 11 May 1904 Lady Elizabeth Anne Blount, who would go on to be influential in the formation of the Flat Earth Society, hired a commercial photographer to use a telephoto lens camera to take a picture from Welney of a large white sheet she had placed, the bottom edge near the surface of the river, at Rowbotham's original position six miles (9.7 km) away. The photographer, Edgar Clifton from Dallmeyer's studio, mounted his camera two feet above the water at Welney and was surprised to be able to obtain a picture of the target, which he believed should have been invisible to him given the low mounting point of the camera. Lady Blount published the pictures far and wide.

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(text Wikipedia)