OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 



221 



for the present purpose, a similar pendulum, No. 13, destined eventually for 

 the Brussels Observatory. . oj iK-iiiuyi. j 



The arrangement of the observations in each of the succeeding experiments 

 was the same as in those already related ; the pendulum was first vibrated in 

 air of full pressure ; then in a rarefied medium ; and lastly, again in the air. 

 A mean was then taken between the two series in air ; with which mean, the 

 intermediate vibration in the i-arefied medium was compared. The result of 

 the comparison was thus wholly independent of the daily rate of the clock ; and 

 in some measure also, of its deviations from an uniform rate in intervals less 

 than 24 hours. The clock was compared daily with the transit clock of the 

 obsei-vatory ; but as the weather in the last half of January and first half of 

 Febraary was very rarely clear, and as the transit clock about that period was 

 more than usually irregular in its going, it has been deemed preferable to take 

 a mean rate for the coincidence clock for the months of January and Febru- 

 ary ; its deviations from this mean rate on the days of experiment ai"e trans- 

 ferred in appearance to the going of the pendulum on different days : this ap- 

 parent in-egularity is however wholly inconsequential in respect to the purpose 

 of the present experiments, for which it is only necessary that the vibrations 

 of the pendulum should be relative in the three series of coincidences forming 

 each distinct experiment. 



Exp. III. — Greenwich, January 14th and 15th. Clock gaining 4™ 14^38. 



The air was then withdrawn, the pendulum again set in motion, and suffered 

 to vibrate an hour before the registry was commenced. 



