250 Mr. Scrymgeouv's Experiments with the Seconds Pendulum 



light pendulum with a wooden bob, of a cylindrical form, about 

 4-i inches long and 2 inches in diameter; this bob was 10 

 ounces in weight, and, along with a brass ball and the pendu- 

 dulum rod, weighed about 15 ounces. A piece of thin iron, 

 about J of an inch in breadth, was fixed to the end of the 

 pendulum rod, edgewise to the direction of motion of the pen- 

 dulum. The object in making it so light was, that the effect 

 of buoyancy might be more readily observed, and also its dif- 

 ference of rate in air and in vacuo. 



The pendulum thus mounted was suspended in the vessel 

 in which the vacuum or exhaustion was to be produced. A 

 small trough of mercury, about 6 inches long, 1 inch deep, and 

 1 inch broad, was placed at the bottom of the vessel, having 

 its length parallel to the direction of motion of the pendulum. 

 After Frequent adjustments and exhaustions, I succeeded in 

 making the lower edge of the piece of iron traverse slightly in 

 the mercury, so that at the end of each vibration it was nearly 

 free of the surface. Some dust upon this surface indicated a 

 slight motion in the direction of the pendulum ; but it almost 

 ceased with the motion of the pendulum at its extent of vibra- 

 tion, and appeared to produce no effect upon the time in de- 

 scending. 



By such means, the pendulum was made to lose its extent of 

 motion nearly at the same rate as a pendulum with a metallic 

 bob does in air; the clock pendulum was adjusted to vibrate 

 with the light pendulum. The trough with the mercury was 

 afterwards removed, the vessel was exhausted to the same de- 

 gree as before, and the same mean extent of vibration was 

 employed ; but careful experiment showed no perceptible dif- 

 ference in time. 



In this experiment, I considered that the mercury should 

 buoy up the pendulum as much as air; and if so, we must evi- 

 dently conclude from the result, that buoyancy makes no dif- 

 ference upon the time of the vibrations of a pendulum. When 

 the same pendulum used above was made to vibrate in air, it 

 lost one vibration in 20 minutes upon the clock pendulum. 



By this time I had begun to suspect the true cause why the 

 pendulum lost time when vibrating in air, and my conclusion 

 was as follows : A current was generated in the air by the 

 motion of the pendulum, and in the direction of its motion; 

 consequently, when the pendulum reached its height and its 

 momentum was exhausted, the current thus generated now 

 slightly suspended its motion, and retarded the first incre- 

 ments of its descent. 



The current of air generated by the vibrations of the bob 

 of a pendulum may be rendered distinctly visible thus : Fix a 



