Jgl|6.] Scientific Notices-^Miscellaneo^s. >i|5 



observations for determining 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 disk 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 disk, would be productive of error. 



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

 respect to the convertible pendulum, or that used for deter- 

 mining the absolute length of the seconds pendulum, the disk 

 was made to subtend 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 disap- 

 pearance and re-appearance is rigorously nothing; an adjust- 

 ment indispensable in his method of observing, when the object 

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



2dly. With the invariable pendulum the disk subtended a 

 somewhat less angle than the tail-piece, so that the inferred 

 number of vibrations in 24 hours was diminished about twp- 

 tenths of a second. But experiments with the invariable 

 pendulum being intended to be in the strictest sense of the 

 word comparative, this constant difference will no way affect 

 the ultimate result. But, as the most direct way to remove any 

 doubts which may be entertained on the subject, the author has 

 computed from the whole of Captain Sabine's observations, the 

 successive differences in the vibrations at the various stations 

 visited by him, by the two methods ; viz. that of employing the 

 disappearances and re-appearances, and the disappearances 

 alone. The results only in one instance differ so much as a 

 tenth of a vibration, they are indifferently in excess and in defect, 

 and the mean of their discrepancies is exactly nothing. From 

 this he concludes that if the observations be made as nearly as 

 possible under similar circumstances, the method of observing 

 by disappearances alone, is productive of no perceptible error 

 in practice, in experiments with the invariable pendulum ; while 

 in those with the convertible pendulum, the equal apparent siz.es 

 of the disk and tail-piece, preclude the possibihty of any, either 

 in practice or theory, from this cause. 



The standard of Sir G. Shuckburgh having been found iden- 

 tical with that by Bird, in the custody of the Clerk of the House 

 of Commons, adopted as the imperial standard unit of exten- 

 sion, the length of the pendulum already determined is fixed 

 with the same degree of precision in parts of the imperial stan4- 

 ard yard. 



;. A repetition of Sir G. Shuckburgh's experiments on the 

 weight of given volumes of distilled water, ^nd ^ re-mejisur^ 



