Perpendicularity of Bunker Hill Monument. 309 



occasion, during a sudden shower, accompanied with strong 

 wind from the southeast, at about three o'clock in the after- 

 noon, to move in the space of a very few minutes a quarter 

 of an inch to the eastward. Observations at seven o'clock 

 in the morning, at twelve o'clock at noon, and at seven 

 o'clock in the afternoon, were recorded through several 

 weeks, and no doubt remains that a cause coincident with 

 the sun in its progress produced the variation of the perden- 

 dicular in the monument. 



A fact already hinted at, further confirmed this conclusion. 

 The extreme departure of the ball from the centre was to 

 the west of northwest ; not to the north, as might at first 

 glance be supposed. The explanation is found in the posi- 

 tion of the monument. Its sides do not face the cardinal 

 points, but are inclined about 20°. The expansion of a single 

 side would produce inclination in a direction perpendicular 

 to the side. The expansion of two adjacent sides would pro- 

 duce inclination in the direction of the diagonal. In the 

 morning the shaft is inclined to the westward. At noon it 

 is inclined but a little to the north of west. In the progress 

 of the afternoon, it sweeps over twice the amount of move- 

 ment in the morning ; describing, in the twelve hours of ob- 

 servation, an arc of an ellipse. 



During the night it sets back to the centre, and before 

 seven o'clock in the morning, has already moved westward. 



The greatest diameter of the irregular ellipse, described 

 by the index in twenty-four hours, is ordinarily less than 

 half an inch, while the least was less than a quarter of an 

 inch. 



It would not be difficult to find the expansion of the granite 

 to which this movement of the ball corresponds. In the 

 simpler case of a rectangular shaft, the departure of the 

 ball from the centre would be the versed sine of an arc (the 

 side of the shaft), of which the pendulum was the sine. The 

 difference between the arc and sine would be the expansion 

 of the granite. 



The heat of the sun penetrates to but a moderate depth. 

 This is evident from the prompt movement of the column 

 when a shower falls only upon the more highly-heated sides, 



