310 Perpendicularity of Bunker Hill Monument. 



and also from the ready change in inclination as the day ad- 

 vances. 



The effects here observed, and which are now recorded 

 from day to day, taken in connection with the meteorologi- 

 cal record of Boston, Charlestown, and Cambridge, cannot 

 fail to be of high interest. 



The expansion of granite by heat had before been observed. 

 Mr Bond, the director of the Cambridge Observatory, noticed 

 its effect on his transit instrument erected in the temporary 

 establishment at the corner of Quincy and Harvard streets. 

 The instrument rested on two granite pillars. In the morn- 

 ing of a clear day, his meridian mark on a distant hill would 

 be found east of the meridian line as indicated by his instru- 

 ment ; at noon, or a little past, coincident with it ; and at 

 evening west of it. 



Engineers have observed it in long walls of masonry. It 

 can scarely be doubted that we have memorials of it in the 

 ruins of Baalbec and Paestum, of Nimrod and Stonehenge; nor 

 can we question that it has played a large part in the de- 

 struction of cliffs, or the splitting of mountain masses. 



The mode of observation at the monument is this : On 

 either side, about three-quarters of a inch from the centre, 

 under the index of the ball, two slender needles have been 

 driven into the floor, leaving not more than the sixteenth of 

 an inch above. These are made by pressure to pierce a card 

 of thin drawing paper, which is kept from warping by slen- 

 der bars of lead. When fixed the north and south and east 

 and west lines are transversed in pencil mark from the floor 

 to the paper. After bringing the ball to rest, in which the 

 observer is aided by a contrivance enabling him to steady 

 his hands, a dot is made with a pencil immediately under the 

 index point, which is about the sixteenth of an inch above 

 the paper. At the close of the day, the card, previously dated, 

 is removed, and another takes its place for the observation 

 of the next day. 



It is a grateful duty to state that the expense of the ne- 

 cessary fixtures at the monument for the pendulum experi- 

 ment, of which advantage has been taken in the observations 

 here referred to, has been incurred bv the Massachusetts 



