336 CAPTAIN SABINE ON THE REDUCTION TO A VACUUM OF 



dulum by their loss of moisture as the weather became more dry. As we now 

 know that the general effect of the air in retarding- the vibration is more than 

 twice as great as was then imagined, and that the wooden tail pieces, in con- 

 sequence of their position, have a far greater proportionate influence on the 

 retardation than would be simply due to the diminution which they occasion in 

 the general specific gravity of the pendulum, we perceive increased reason to 

 agree with Captain Kater, and to apprehend so much danger of derangement 

 from this cause, as to make it desirable to avoid altogether the employment 

 of a material susceptible of changes from moisture. 



For this reason I determined to substitute tail pieces of brass, with such 

 an alteration in the position or size of the smaller weight as should re-esta- 

 blish the equality of vibration. Previously, however, to this being done, and 

 for the purpose of illustrating more strongly the effect of the wooden tail 

 pieces, I had them reduced to less than half their original length, 10.6 inches 

 being taken off from each, leaving them 6.4 inches from the extremity of the 

 brass bar. By substituting a still smaller weight (of 1 925 grains) of the same 

 metal and form, for the smallest weight used by Captain Kater (of 3325 

 grains), and securing it very nearly in the same position, the pendulum was 

 again rendered nearly convertible, and experiments were made with it in the 

 same succession as before, of which the following are the results. 



With the great weight below, there was found a difference of 11.9 vibra- 

 tions per diem, corresponding to a difference of atmospheric pressure of 28.741 

 inches of mercury at 32°, the temperature of the air of full pressure being 53.2, 

 and of the rarefied air 53.8. 



And with the great weight uppermost a difference of 14 vibrations per 

 diem, corresponding to a difference of 28.16 inches of mercury at 32°, the 

 temperature of the air of full pressure being 51.4, and of the rarefied air 53.8. 

 Whence we obtain 12.4 vibrations per diem as the reduction to a vacuum, 

 when the wooden tail pieces were shortened from 17 inches to 6.4 inches, and 

 the great weight was below, in air of 53°.5, under a pressure of 30 inches of 

 mercury at 32° ; and 14.9 vibrations per diem with the great weight uppermost, 

 in air of the same temperature and density. 



The wooden tail pieces were then altogether removed, and slips of brass 

 substituted, extending 7 inches from each extremity of the bar. The pendulum 



