381 



his brother-in-law, the late Mr. Shaw, for the valuable assistance 

 he afforded him in the whole course of his investigations on the 

 nerves. 



On the Reduction to a Vacuum of Captain Kater's convertible Pendu- 

 lum. By Captain Edward Sabine, of the Royal Artillery, Secretary 

 to the Royal Society. Read June 18, 1829. [Phil. Trans. 1829, 

 p. 331.] 



Recent investigations having shown that the method employed by 

 Captain Kater for the reduction of his experiments on the length of 

 the pendulum vibrating seconds in air, to that of the same pendulum 

 in vacua, was founded on erroneous principles, the author undertook 

 to ascertain, by direct experiment, the actual difference of the num- 

 ber of vibrations of the pendulum employed by Captain Kater, in air 

 of ordinary density, and in highly rarefied air. The alteration of 

 density in the medium in which the pendulum is swung, would, in 

 the first place, if its form were not symmetrical, affect its converti- 

 bility ; that is, the same adjustment of the axes which gave an equa- 

 lity of oscillations in reversed positions, when vibrating in air, would 

 not afford the same equality in a more rarefied medium. It follows 

 also, from the corrected investigation, that the amount of the retar- 

 dation occasioned by the air is considerably greater than what had 

 been originally computed from the simple consideration of buoyancy. 



These inferences have been fully confirmed by the experiments of 

 Captain Sabine. The increase in the number of vibrations per diem 

 with the convertible pendulum as it was used by Capt. Kater, that 

 is, vibrating with the great weight below, in vacua, above those in 

 air of the temperature of 49, under a pressure of 30 inches of mer- 

 cury at 32, was 15 '71 : when inverted, the other conditions remain- 

 ing the same, the increase was 16' 13 vibrations per diem. 



Captain Kater had observed that considerable changes in the hy- 

 grometric state of the atmosphere destroyed the convertibility of his 

 pendulum, from their affecting the weight of the pieces of wood at 

 both of its ends. In order to remove this source of error, and also 

 to ascertain its amount, the author first reduced the wooden tail- 

 pieces from 17 inches, their original length, to 6'4 inches. The in- 

 crease of the number of vibrations was then, with the great weight 

 above, 14*91, and with the great weight below, 12'41 per diem. 

 When the wooden tail-pieces were wholly removed, and slips of brass 

 substituted for them, the increase was further reduced, in like cir- 

 cumstances, to 12'83 in the former case, and 11 '58 in the latter. 



