of minute Forces on the Rate of the Seconds Pendulum. 349 



Set, No. 4. 



_ . Loss in Extent of 



Experiments. g4 Hours Vibration. 



In air -2 m I s 2° 



In exhaustion, 1 inch —1 55 2 



These results show that the varnishing of the bob made 

 no difference in the time of the vibrations. The apparent 

 difference between the results in air and in exhaustion arose 

 from the bob not being set precisely in its former place after 

 varnishing. 



The following experiments were made in the open air, with 

 the view of ascertaining the differences in time in the differ- 

 ent arcs of vibration. 



Set, No. 5. 



r, . Loss in Extent of 



Experiments. 04 Hours. Vibration. 



In air -2 m 7'6 S 2-0° 



Ditto.. -2 12-3 2-5 



Ditto —2 18-2 3-0 



Ditto -2 24-2 3-5 



Between the first experiments of Set, No 4. and Set No. 5. 

 there is a difference of 6*6 seconds, although the extent of vi- 

 bration was the same. I found afterwards that this was occa- 

 sioned by the want of firmness in the fixture on which the 

 pendulum vibrated, owing to its removal from the exhausted 

 vessel. For although the fixture appeared to be firm, I con- 

 sidered it advisable to increase its stability by an additional 

 fixture; this produced an acceleration in its rate of about 6 

 seconds in 24 hours. Thus it appears that a small change in 

 the stability of a fixture will often produce a great effect on 

 the rate of a pendulum. Owing to the slight instability of 

 the fixture, in the above experiments, the difference in the 

 times of vibration in the arcs of 2° and 3 0, 5 may be two or 

 three seconds greater than it ought otherwise to be. 



The next set of experiments were made for the purpose of 

 ascertaining the effect of resistance or friction on the rate of a 

 pendulum, in the case where no current was generated by the 

 resistance. 



The mercurial pendulum, having a suspending spring so 

 adjusted as to cause the long and short vibrations to be nearly 

 isochronous, was employed. A hair was stretched parallel to 

 the motion of the pendulum so as touch the rod slightly, about 

 2 inches below the point of suspension. 



