255 



it U entitled to. For this purpose it is necessary to compare thin 

 final result with those obtained in other experiments and by different 

 methods. Now it appears that previous to the experiments detailed 

 in the author's paper on the subject in the Philosophical Transactions 

 for 1818, on which this result rests, another series is there men- 

 tioned, made with the same instruments, but under circumstances 

 which occasioned their rejection, and which, owing to some repairs in 

 the instruments between the two series, which occasioned a material 

 alteration in the distance between the knife edges, have all the weight 

 of experiments made with a different pendulum ; the result of these 

 rejected experiments, however, differed only two ten-thousandths of 

 an inch from that ultimately adopted. 



The author next compares the length of the seconds pendulum at 

 Unst and at Leith Fort, as ascertained by him by an invariable pen- 

 dulum, whose vibrations had previously been determined in London, 

 and whose length was thus known in terms of the London Seconds 

 Pendulum, and as ascertained by M. Biot at the same stations, by 

 means of a variety of pendulums, and by a totally different method 

 of observation, that of Borda. The results of this comparison are 

 a difference between the determinations of M. Biot and the author, 

 of 0-00029 inch in excess at the former station, and O'OOOIS in 

 defect at the latter. 



From this near agreement of all the results, he considers that the 

 length of the seconds pendulum in London, may be regarded as cer- 

 tainly known to within one ten-thousandth of an inch ; while from the 

 near agreement of the results of the French and English experiments 

 on the length of the pendulum, he concludes that the length of the 

 metre in parts of Sir George Shuckburgh's scale, may also be re- 

 garded as known within one ten-thousandth of an inch. 



From an account recently published by Captain Sabine of his va- 

 luable experiments for the determination of the variations in length 

 of the seconds pendulum, he observes, doubts may be inferred of the 

 accuracy of the method employed by him for the observations for de- 

 termining the length of the seconds pendulum in London, as well as 

 in those which have been made with the invariable pendulum. It is 

 asserted there, that taking a mean between the disappearances and 

 re-appearances of the disc, is a more correct method of observation 

 than than pursued by Captain Kater, and that the intervals between 

 the coincidences obtained, by observing the disappearances only of 

 the disc, would be productive of error. 



In answer to this objection the author remarks, 1st, That with re- 

 spect to the convertible pendulum, or that used for determining the 

 absolute length of the seconds pendulum, the disc was made to sub- 

 tend precisely the same angle as the tail-piece of the pendulum, so 

 that at the moment of disappearance its centre necessarily coincided 

 precisely with the middle of the tail-piece, and the difference between 

 the moments of disappearance and re-appearance is rigorously no- 

 thing, an adjustment indispensable in his method of observing, when 

 the object is to determine the true number of vibrations in 24 hours. 



