464 



regarded as representing the joint effects of the expan- 

 sion of the metal, and of the difference of temperature 

 on that part of the reduction to a vacuum which is not 

 comprehended in the ancient " correction for buoyancy." 

 (Phil. Trans., 1829, p. 238.) 



Although the correctness or incorrectness of the mea- 

 surements of the absolute length of the pendulum, 

 which were made before 1828, has no bearing on the 

 ellipticity derived, as here, from direct experiments on 

 the acceleration, it may be well to notice the great com- 

 parative influence which Bessel's discovery has on one 

 at least of the methods, by which the true length of the 

 seconds pendulum was previously supposed to have been 

 determined ; whence we may at once perceive the inse- 

 curity of inferring supposed comparable values of the 

 acceleration from absolute lengths measured in different 

 places by different methods. Experiments made in 1830, 

 and reported in the Phil. Trans, for 1831, art. xxv., with 

 the vacuum apparatus already spoken of, and with the 

 identical convertible pendulum which had been used by 

 Captain Kater (in his experiments on the length of the 

 seconds pendulum in London in 1817), showed that 

 the true reduction to a vacuum of that pendulum 

 was more than double the amount calculated by the 

 formula since known to be erroneous. The true reduc- 

 tion was between 12 and 13 seconds in the 24 hours, 

 instead of about 6 seconds, as computed and employed 

 by Captain Kater; the error in his determination was 

 consequently more than 6 seconds in a day, equivalent 

 to '006 parts of an inch in the length of the seconds 

 pendulum. In the case of the convertible pendulum, 



