CAPTAIN rater's CONVERTIBLE PENDULUM. 337 



was rendered convertible in air by the small weight used by Captain Kater, 

 screwed to the bar at a somewhat greater distance from its knife edge than in 

 his experiments ; and coincidences being observed in the usual routine, it 

 appeared that with the great weight below, a difference took place of II .3 vibra- 

 tions per diem, corresponding to a difference of atmospheric pressure of 28.46 

 inches of air at 32°, the temperature of the air of full pressure being 58°, and 

 of the rarefied air 55°.5 : and with the great weight uppermost, a difference of 

 12 vibrations per diem, cori'esponding to a difference of atmospheric pressure 

 of 28.04 inches of air at 32°, the temperature of the air of full pressure being 

 60°, and of the rarefied air 60°.5. 



Whence we obtain when the great weight is below, 1 1 .8 vibrations per diem 

 as the reduction to a vacuum, in air of 60°, under a pressure of 30 inches of 

 mercury at 32° ; and 12.8 vibrations per diem when the great weight is upper- 

 most. 



Finally, recapitulating the results obtained with* the different modifications 

 of the tail pieces, we have 



1. With the wooden tail pieces 1 ^ ^ • v, ^ u Citation.. vibration,. 



17 inches in length ! . . . j ^'^^^ ^^^^^^ ^^ove 16.1 ; below 15.7 



2. With the wooden tail pieces 1 



6.4 inches in length . . . . j 



14.9; 12.4 



3. With the brass tail pieces 7 1 ,o q 



inches in length J ^^'^' ^^'^ 



In comparing Nos. 1 and 2 with each other, and both with No. 3, we perceive 

 the very great effect which the employment of so light a material as wood for 

 the tail pieces of the pendulum produces, in increasing the difference between 

 the vibrations in air and those in a vacuum. When slips of brass were sub- 

 stituted for the slips of deal employed by Captain Kater, the retardation 

 caused by the air was diminished not less than between 3 and 4 vibrations per 

 diem on the one knife edge, and upwards of 4 vibrations per diem on the 

 other. When the wooden tail pieces were reduced to the same length, or 

 nearly so, as those of brass, the retardation with them was still found greater 

 than with the brass tail pieces of similar size and figure, by 2 vibrations per 

 diem on the one knife edge, and 1 vibration per diem on the other. The re- 

 tardations were in all cases much more considerable than would have been 



MDCCCXXIX. 2 X ^ " 



* ., 



