TOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 3g3 



but the motion then increased, so that, in about a quarter or half an hour more, 

 it was found to have moved 4. or 14- division, in the same direction that it ought to 

 have done by the action of gravity. On returning the irons back to their former 

 position, the arm moved backward, in the same manner that it before moved for- 

 ward. It must be observed, that the motion of the arm, in these experiments was 

 hardly more than would sometimes take place without any apparent cause; but yet, 

 as in 3 experiments which were made with these rods, the motion was constantly of 

 the same kind, though differing in quantity from 4 to 14. division, there seems 

 great reason to think that it was produced by the rods. As this effect seemed to 

 be owing to magnetism, though it was not such as I should have expected from 

 that cause, I changed the iron rods for copper, and tried them as before; the result 

 was, that there still seemed to be some effect of the same kind, but more irregular, 

 so that I attributed it to some accidental cause, and therefore hung on the leaden 

 weights, and proceeded with the experiments. It must be observed, that the effect 

 which seemed to be produced by moving the iron rods from one near position to the 

 other, was, at a medium, not more than one division ; whereas the effect produced 

 by moving the weight from the midway to the near position, was about ] 5 divi- 

 sions; so that, if I had continued to use the iron rods, the error in the result thus 

 caused, could hardly have exceeded -3V of the whole. 



In exper. 1, Aug. 5, the motions of the weights between the midway and posi- 

 tive positions were thus: 



Motion on moving from midway to positive = 14.32 



From positive to midway = 14.1 



Time of one vibration =± 14 m 55 s . 



It must be observed, that in this experiment, the attraction of the weights drew 

 the arm from 11.5 to 25.8, so that, if no contrivance had been used to prevent it, 

 the momentum thus acquired would have carried it to near 40, and would there- 

 fore have made the balls strike against the case. To prevent this, after the arm 

 had moved near 15 divisions, I returned the weights to the midway position, and 

 let them remain there, till the arm came nearly to the extent of its vibration, and 

 then again moved them to the positive position, by which the vibrations were so 

 much diminished, that the balls did not touch the sides ; and it was this which pre- 

 vented my observing the first extremity of the vibration. A like method was used 

 when the weights were returned to the midway position, and in the 2 following ex- 

 periments. The vibrations, in moving the weights from the midway to the positive 

 position were so small, that it was thought not worth while to observe the time of 

 the vibration. When the weights were returned to the midway position, I deter- 

 mined the time of the arm's coming to the middle point of each vibration, in order 

 to see how nearly the times of the different vibrations agreed together. In great 

 part of the following experiments, I contented myself with observing the time of 

 its coming to the middle point of only the first and last vibration. 

 vol. xviii 3 E 



