SOS Professor Horsford on the Effect of Heat on the 



The Effect of Heat on the Perpendicularity of Bunker Hill 

 Monument. By Prof. E. N. HoiiSFORD, of Harvard, North 

 America. 



Soon after the pendulum was placed in Bunker Hill Monu- 

 ment, it was observed that the ball when at rest was not 

 always over the same point in the floor. The careful con- 

 sideration of all the conditions of this fact resulted in ascrib- 

 ing it to the unequal expansion of the sides of the monument, 

 in consequence of unequal exposure to the sun. 



A brief description of the present condition of the monu- 

 ment will aid in understanding the mode of observation 

 pursued. 



The obelisk, thirty feet square at the base, rises, gradually 

 lessening, to a pyramidal summit, two hundred and twenty- 

 one feet. Within is a circular well, seven feet in diameter 

 at the bottom, and five at the top, where it opens into a 

 chamber or observatory. The chamber is approached by a 

 winding stairway. In the centre of the roof of the chamber 

 is an iron staple which was securely fixed at the time of 

 placing the capstone. It served at first to support machinery 

 for carrying visitors up and down. From this staple, which 

 is over the centre of the open space or well, the pendulum is 

 suspended by means of a screw clamp. 



From a point in the floor directly below the index attached 

 to the ball, circles were described and graduated, and radii 

 drawn. 



On the day following the graduation, the index was found 

 to be on the one side of the centre of the circle. As the 

 screw clamp first employed did not admit of adjustment, a 

 new apparatus, with the necessary modifications, were sub- 

 stituted, and the ball brought precisely over the centre of the 

 graduated circle. 



A few hours later, it was found out of the centre. 



Upon observing more carefully, it was found during clear 

 days that the motion of the ball in the morning was to the 

 westward, at noon to the northwest, and at evening to the 

 east. It was further observed that on days when the sun 

 was obscured by clouds, that no motion of the ball or its in- 

 dex point occurred. It was still further observed on one 



