of minute Forces on the Rate of the Seconds Pendulum. 4-37 



be made about half an inch longer, when vibrating by a strong 

 suspending spring, than when vibrating on a knife-edge, in 

 order that the vibrations may be performed in the same time. 



In the next set of experiments, a suspending spring was em- 

 ployed, which was thinned so much as to be just sufficient to 

 bear the weight of the pendulum. This was done as a pre- 

 paratory step, previous to determining an extent of vibration, 

 such that the amount of the increase of vibration in vacuo 

 would be equivalent to the amount of the loss occasioned by 

 the current generated in air. A cylindrical bob, of 8 pounds 

 weight and 2 inches diameter, was first employed, which, it 

 was supposed, would have nearly the same momentum or spe- 

 cific gravity, as the common mercurial pendulum with its glass 

 vessel. A piece of thin tin-plate was fixed at the lower end, 

 across the centre of the bob, and projecting downward l£ inch ; 

 and the pendulum was so constructed, that the bob could be 

 turned by a swivel, without altering its length or removing it 

 from the vessel. By this means the position of the piece 

 of tin-plate could be altered, so as to be either edgewise or 

 broadside to the path of the pendulum. The piece ot tin-plate 

 would thus produce a greater or less resistance, as well as a 

 greater or less counter current to the motion of the pendulum, 

 with the same weight, accordingly as it was placed broadside 

 or edgewise. 



Set, No. 8. — With the tin-plate placed broadside to the path 



of the pendulum, the impulse being given equally in ascent 



and descent. 



„ • Loss in Extent of 



Experiments. 24Hours VibratJon 



In air -6 m 19 8 20° 



Ditto -6 19-6 2-0 



Ditto -6 18-6 2-0 



In exhaustion, 6 in. . . —6 15*8 2*8 



Ditto 13J . . . -6 16*2 



In air -6 19-2 2*0 



Ditto -6 19-5 2-0 



Set, No. 9. — With the tin-plate turned edgewise to the path 

 of the pendulum. 



J, . . Loss in Extent of 



Experiments. 24 Hours. Vibration. ( 



In air — 6 m 18'5 g 2-2° 



Ditto —6 19*0 2-2 



In exhaustion, 6 in. . . —6 185 2*85 



Ditto 19£ . . -6 19-2 26 



In air -6 192 2*2 



