Foucault’s Experiment
Seven years later (1850), two Harvard professors checked with elaborate apparatus, paid for by members of the Bunker Hill Monument Association and the Massachusetts Charitable Association, the famous experiment of Foucault which used a long pendulum to prove the daily rotation of the earth. Suspended in the newel by an annealed wire, 210 feet long, the oscillations of a pointer, attached to a 31-pound British cannon ball relic of the battle, were observed; and its plane of swing was seen to revolve during the day from right to left of the observer. A sudden shower on a previously bright day complicated the experiment, until Professor Eben N. Horsford discovered the reason. Cooled by the rain the monument’s exposed face contracted; its apex moved correspondingly and carried the point of suspension of the pendulum with it. As observed years later on the Washington Monument, Horsford deduced that the side of an obelisk, exposed to the hot sun, expanded, and that its apex followed the sun in the sun’s travel from east to west. Such motion is tiny, and the ingenuity of the apparatus to observe it was notable. The path of the orbit of the bob, registering both the earth’s rotation and the effect of varying heat on the monument’s sides, was described as an irregular ellipse with major axis of one-half an inch.