300 Proceedings of the Royal Society. 



seconds' pendulum, and as ascertained by M. Biot at the same sta- 

 tions by means of a variety of pendulums, and by a totally differ- 

 ent method of observation — that of Borda. The results of this 

 comparison are, a difference between the determinations of M. B. 

 and of the author, of 0.00029 inches in excess at the former sta- 

 tion, and 0.00015 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 certainly 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 G. Schuckburgh's scale 

 may also be regarded as known within one ten- thousandth of an 

 inch. 



From an account recently published by Captain Sabine of his 

 valuable experiments for the determination of the variations in 

 length of the seconds' pendulum, he observes, doubts may be in- 

 ferred of the accuracy of the method employed by him for the ob- 

 servations for determining the length of the seconds' pendulum in 

 London, as well as in those which have been made with the inva- 

 riable pendulum. It is asserted there, that taking a mean be- 

 tween the disappearances and ie-appearances of the disc is a more 

 correct method of observation than that pursued by Captain 

 Kater, and that the intervals between the coincidences obtained, 

 by observing the disappearances only, of the disc, would be pro- 

 ductive of error. 



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

 respect to the convertible pendulum, or that used for determining 

 the absolute length of the seconds' pendulum, the disc was made 

 to subtend precisely the same angle as the tail-piece of the pen- 

 dulum, so that at the moment of disappearance, its centre necessa- 

 rily coincided precisely with the middle of the tail-piece, and the 

 difference between the moments of disappearance and re-appear- 

 ance is rigorously nothing ; an adjustment indispensable in his 

 method of observing, when the object is to determine the true 

 number of vibrations in 24 hours. 



