of minute Forces on the Hate of the Seconds Pendulum. 441 

 Set, No, 21. — With a diminished maintaining power. 



In air 4- 5'9 S 2*3° 



Ditto + 6*3 2'3 



In exhaustion, 6j in. + 7*4 2*7 



Ditto 18 . . . +7*0 2-6 



In air +6*2 2'3 



Ditto +60 2-3 



In these experiments, the exhaustions were carried as far as 

 the pump was effective ; the float-gauge generally gave indica- 

 tions within two or three tenths of the barometer, but the ex- 

 haustions were probably rather greater than what were indi- 

 cated, as the surfaces of the mercury could not be seen, iron 

 pipes being used for the gauge. The mean exhaustions de- 

 tailed in the various experiments were taken at the end of 12 

 hours, and extended to 24, in the tables. The number of 

 pieces of which the vessel was composed, and the necessity of 

 frequently opening it, rendered it a matter of great difficulty 

 to keep it air-tight. At the place where the pendulum bob 

 traversed, a horizontal section of the vessel was 13 inches long 

 and 9 inches broad. 



From these experiments, it appears that if a pendulum with 

 a mercurial or a cylindrical bob be made to vibrate at the 

 extent of 2 0, 2 or 2°-3 from the point of rest, the time of its 

 vibrations will not be altered by changes in the density of the 

 atmosphere ; and the same observation holds good with t 

 common leaden bob, vibrating to an extent of 2°*6 or 2°*7 

 The reason of this is, that the gain resulting from the weak- 

 ened current is compensated by the loss arising from the in- 

 crease of vibration. The following conditions are, however, 

 necessary for the production of this result: the impulse must 

 be given in the middle (that is, one half in descent, and one 

 half in ascent); and the pendulum must be suspended by a 

 very thin spring, or on a knife-edge. 



Though an adjusted spring would compensate for the loss 

 of vibration arising from increase of friction or decrease of 

 maintaining power, and would be but little affected by changes 

 in the atmosphere, yet it is liable to changes in its rate arising 

 from the weakening of the spring by continued action ; at least 

 this is generally the case for several years after it is first set 



g° in g- 



From these considerations^ appears that a weak suspending 



spring or knife-edge, adjusted so as to admit of an increase 



in the extent of vibration as the density of the air decreases, 



Third Series. Vol. 2. No. 12. June 1833. 3 L 



