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. _Today_ Two hundred thousand persons visit Bunker Hill every year. Of these visitors 20,000 pay their $0.10, and presumably climb the 294 granite steps to the top.[12] Very few Bostonians are among these visitors.[13] They are of various types: honeymooners and casual tourists, whose list of the sights to be seen in historic Boston includes the monument, and historically minded youngsters, one of whom was recently caught in a heated argument with his father as to where the order was to “wait ’til you can see the whites of their eyes.” Surely, the stout young man who recently lugged 25 pounds to the top--his young daughter--was not typical. [12] Estimate of custodian from a review of his register, 1951. [13] “Those pangs of conscience he feels every June 17 are as close as the average Bostonian ever comes to climbing Bunker Hill Monument.”--_Boston Globe_, 18 June 1951. The evidence that the monument is probably the most popular of the historic shrines of old Boston would be as pleasing to the charter members of the Bunker Hill Monument Association as to the present members of this venerable society, which lists among present and past members, 30 generals, 12 admirals, 12 Presidents of the United States, a score of Massachusetts governors, 20 mayors of Boston, and 6 presidents of Harvard University. It is a healthful society to belong to, the 1949 _Proceedings_ say, in the mention of 46 half-century members of whom 16 were then living.