VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 397 



May 22, 17Q8. The experiment was repeated in the same manner, except that 

 the lamps were made so as to burn only a short time, and only 2 hours were suffered 

 to elapse before the weights were moved. The weights were now found to be 

 scarcely 2° warmer than the case ; and the arm was drawn aside about 2 divisions 

 more, after the weights had remained an hour in the position they were moved to, 

 than it was at first. 



On May 23, the experiment was tried in the same manner, except that the 

 weights were cooled by laying ice on them ; the ice being confined in its place by 

 tin plates, which, on moving the weights, fell to the ground, so as not to be in the 

 way. On moving the weights to the negative position, they were found to be 

 about 8° colder than the air, and their effect on the arm seemed now to diminish 

 on standing, instead of increasing, as it did before ; as the arm was drawn aside 

 about 2-^ divisions less, at the end of an hour after the motion of the weights, than 

 it was at first. 



It seems sufficiently proved therefore, that the effect in question is produced, as 

 above explained, by the difference of temperature between the weights and case ; 

 for, in the 6th, 8th, and 9th experiments, in which the weights were not much 

 warmer than the case, their effect increased but little on standing ; whereas it in- 

 creased much when they were much warmer than the case, and decreased much 

 when they were much cooler. It must be observed, that in this apparatus the box 

 in which the balls hang must be near the bottom of it, which makes the effect of 

 the current of air more sensible than it would otherwise be, and is a defect which 

 I intend to rectify in some future experiments. v 



After this were made 3 other experiments, with the weights first in the positive 

 and then moved to the negative position ; viz. exper. 9 on April 29, exper. 10 on 

 May 5, and exper. 1 1 on May 6 ; the results of which were as follow : viz. 



April 29. Motion of arm = 6.32 



Time of vibration = 6 m 58 s 



May 5. Motion of arm as 6.15 



Time of vibration = 6 m 59 s 



May 6. Motion of arm = 6.07 



Time of vibration = 7 m I s 



In the foregoing 3 experiments, the index was purposely moved so that, before 

 the beginning of the experiment, the balls rested as near the sides of the case as 

 they could, without danger of touching it ; for it must be observed, that when the 

 arm is at 35, they begin to touch. In the following 2 experiments, the index was 

 in its usual position. Next follow 3 more such experiments, by varying the posi- 

 tions from negative to positive, and the contrary; viz. exper. 12, 13, 14, on May 

 9, 25, 26 respectively ; the results of which are as follow . viz. 



Exper. 12. Motion of arm = 6.09 



Time of vibration = 7 m 3 s 



