3Q4 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



In the 2d experiment, Aug. 6, in like manner, the time of 1 vibration was 

 14 m 42 s ; and in the 3d experiment, Aug. 7, it was 14 m 46 s . 



These experiments are sufficient to show, that the attraction of the weights on 

 the balls is very sensible, and are also sufficiently regular to determine the quantity 

 of this attraction pretty nearly, as the extreme results do not differ from each other 

 by more than ~ P art « But there is a circumstance in them, the reason of which 

 does not readily appear, namely, that the effect of the attraction seems to increase, 

 for half an hour, or an hour, after the motion of the weights; as in all the 3 expe- 

 riments, the mean position kept increasing for that time, after moving the weights 

 to the positive position ; and kept decreasing, after moving them from the positive 

 to the midway position. The first cause which occurred to me was, that possibly 

 there might be a want of elasticity, either in the suspending wire, or something it 

 was fastened to, which might make it yield more to a given pressure, after a long 

 continuance of that pressure, than it did at first. To put this to the trial, I moved 

 the index so much, that the arm, if not prevented by the sides of the case, would 

 have stood at above 50 divisions, so that, as it could not move farther than to 35 

 divisions, it was kept in a position 15 divisions distant from that which it would na- 

 turally have assumed from the stiffness of the wire; or, in other words, the wire 

 was twisted 1 5 divisions. After having remained 2 or 3 hours in this position, the 

 index was moved back, so as to leave the arm at liberty to assume its natural 

 position. 



It must be observed, that if a wire is twisted only a little more than its elasticity 

 admits of, then, instead of setting, as it is called, or acquiring a permanent twist 

 all at once, it sets gradually, and when left at liberty it gradually loses part of that 

 set which it acquired ; so that if, in this experiment, the wire, by having been kept 

 twisted for 2 or 3 hours, had gradually yielded to this pressure, or had begun to set, 

 it would gradually restore itself, when left at liberty, and the point of rest would 

 gradually move backwards: but though the experiment was twice repeated, I could 

 not perceive any such effect. 



The arm was next suspended by a stiffer wire : after which, in the next 2 expe- 

 riments, being the 4th and 5th, the times of vibration were thus: viz. 7 m 2 s and7 m 

 5 8 . In the 4th experiment, the effect of the weights seemed to increase on standing, 

 in all the 3 motions of the weights, conformably to what was observed with the 

 former wire ; but in the last experiment the case was different ; for though, on 

 moving the weights from positive to negative, the effect seemed to increase on 

 standing, yet on moving them from negative to positive, it diminished. 



My next trials were, to see whether this effect was owing to magnetism. Now, 

 as it happened, the case in which the arm was inclosed, was placed nearly parallel 

 to the magnetic east and west, and therefore, if there was any thing magnetic in 

 the balls and weights, the balls would acquire polarity from the earth ; and the 

 weights also, after having remained some time, either in the positive or negative 



