CAPTAIN KATER’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 1 1 .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, corresponding 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 . 
2 . 
3. 
With the wooden tail pieces ') 
17 inches in length . . . ./ 
With the wooden tail pieces \ 
6.4 inches in length . ... } 
With the brass tail pieces 7 \ 
inches in length J 
Vibrations. Vibrations. 
Great weight above 16.1 ; below 15.7 
14.9; 12.4 
12 . 8 ; 11.8 
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 
