Alex. Gerard, Esq., on Pendulum Observations. 15 



direction. Its position now was farthest west about 11 A.M., 

 and farthest east about 7 P.M., so that 3 P.M. might be con- 

 sidered the culminating point. 



The space passed over was not very accurately noted, but 

 was greatest on days of bright sunshine, being on one occa- 

 sion as much as } inch. 



The room where the experiment was conducted was on 

 the first floor of a strong granite building, and separated 

 from the front by a large apartment and a passage. The sun 

 at that season of the year never shone upon it, and the changes 

 took place irrespective of the temperature of the room. 



While engaged in these observations, I happened to read 

 in a newspaper an imperfect account of M. Foucault's experi- 

 ment, and erroneously identified it with my own. Under 

 this impression, I communicated the results of my observa- 

 tions to Professor Airy, suggesting one or other of the two 

 following explanations ; either — 1st, That, by unequal ex- 

 pansion, the position of the building is in some very small 

 degree altered by the heat of the sun ; or, 2d, That the solar 

 rays produce some very minute change on the direction of 

 gravity. The learned Professor, after a short time, favoured 

 me with his reply, to the effect that, in his opinion, there 

 could not be a change in the direction of gravity to the ex- 

 tent indicated, as this would be inconsistent with the steadi- 

 ness of the zenith point in the best instruments. 



Considering the Astronomer Royal as the most competent 

 judge in this matter, I abandoned the idea of the possibility 

 of such a change ; and the subject had almost dropped from 

 my mind, when my attention was recalled to it by the perusal 

 of your last number. The American Professor joins with 

 our illustrious countryman in ascribing the phenomena to un- 

 equal expansion. Yet, with all deference to so high autho- 

 rity, I would venture to suggest the query, Whether the 

 suspected change of gravity may not lurk as a residuum in 

 some of the small corrections usually made on astronomical 

 observations, and whether the same change may not be the 

 cause of the difficulty experienced in determining latitudes 

 with great precision in the Trigonometrical Survey ? 



With respect to the Bunkers' Hill experiment, it is difficult 



