of minute Forces on the Rate of the Seconds Pendulum, 435 



of the boards was left open. The detached pendulum being 

 previously adjusted to the clock pendulum, at an extent of vi- 

 bration of 2°, the pendulum was put in motion, -j^th of a de- 

 gree beyond. 2°; and though the extent of vibration decreased 

 by 1° in 15 minutes, no difference in the times of vibration 

 was perceivable. When the boards were placed at the distance 

 of \ an inch from the balls, the extent of vibration decreased 

 by T 8 (j ths of a degree in the same time; and when they were 

 removed altogether, it decreased by y^ths of a degree. 



The experiment with the boards at the distance of \ an inch 

 from the balls was repeated, with this difference, that boards 

 were placed at the ends of the former within \ an inch of the 

 extent of vibration on each side of the vertical. Though the 

 space in which the vibrations were performed was thus in- 

 closed, no perceptible difference was observed in the times of 

 vibration, owing to their rapid decrease ; if there were how- 

 ever any difference, it must have been very small. 



The preceding experiments prove that the vibrations of a 

 clock pendulum are not altered in their time, though they may 

 be altered in their extent by the approach of any body, provided 

 the impulse be given in the middle; that is, as much in de- 

 scent as in ascent, with the exception of the alteration which 

 results from the diminished extent of vibration. If, however, 

 the impulse be given in ascent, the times of vibration will be 

 slower, and if given in descent, they will be quicker than if 

 given in the middle ; and these different conditions will alter 

 the times only in proportion to the amount of friction or of re- 

 sistance. 



In the next set of experiments with the clock pendulum, 

 the pallets were adjusted so as to give three fourths of the im- 

 pulse in descent. The suspending spring was also adjusted 

 so as to cause the long and short vibrations to be performed in 

 the same time, when in a detached state, and when furnished 

 with a leaden oval bob of 7 pounds weight. The clock was 

 fitted with pendulum and other apparatus in the vessel in 

 which the exhaustion was to be made. On the top of this 

 vessel there was a glass receiver about a foot deep and 7 inches 

 wide. In the receiver was placed the clock, furnished with a 

 small temporary dial, to show minutes and seconds. 



By this apparatus, the time and the proper adjustment of 

 its different parts, such as the pendulum, suspension, &c. could 

 be observed. 



The glass vessel above mentioned is the same that was em- 

 ployed in all the experiments made with the pendulum vibra- 

 ting in vacuo, or in a state of considerable exhaustion. 



3 K2 



