The Montlhéry Tower

The tower on the hill is the keep of Montlhéry, south of Paris, all that still stands of a medieval castle, and in the 19th century scientists put it to work. One night in 1822 the astronomer François Arago and his colleagues fired a cannon from its top, while observers at Villejuif, about 18 kilometres away, timed the gap between the flash and the bang to measure the speed of sound. Corot's small canvas leaves all that out. A thin young tree twists up in front, a figure in pink passes a white wall, and the tower sits pale against the sky. The Louvre dates it only to the third quarter of the century. In 1874 the physicist Alfred Cornu aimed a beam of light from the Paris Observatory at this same tower to measure the speed of light.