48 Royal Society. 



difference in length of the seconds' pendulum in London and 

 Paris ; by Capt. E. Sabine, Roy. Art., F.R.S." 



The author commences this paper by a brief statement of 

 the existing state of the determinations of standards of length 

 in the two countries ; and he observes that an attempt made by 

 M. Arago in 1817 and 1818, to bring into immediate com- 

 parison the standards of the two countries, proved inconclu- 

 sive, from the rates of the pendulums not having been obtained 

 with sufficient exactness. The author having obtained from 

 His Grace the Master- General of the Ordnance a general leave 

 of absence from his military duties, so long as he could be use- 

 fully employed in scientific pursuits, conceived he could in no 

 way better satisfy the condition than in carrying into effect this 

 purpose. Accordingly, being provided with two pendulums, — 

 one made by M. Schumacher, another the property of the Board 

 of Longitude, — he set out for Paris, whither the pendulums were 

 forwarded to him. The comparison was made in Paris at the 

 Royal Observatory, in the Salle de la Meridienne, on the spot 

 in which M. Biot's measurement had been made, and every 

 proper facility and assistance was afforded him. The coinci- 

 dence clock was compared every twelfth hour by M. Mathieu, 

 with the transit clock of the Observatory. On the 27th of April, 

 the weather having set in mild and steady, the experiments 

 were begun. The results are stated in the tbrm of appended 

 tables, of which a detailed account is given. 



Each of the pendulums, when not used in observing co- 

 incidences, was employed in determining its rate by a journey- 

 man-clock or counter, — amethod used by Messrs. Freycinet and 

 Duperrey ; but which the author thinks inferior to that of co- 

 incidences, though capable of giving good results. The par- 

 ticulars of these are given in two of the tables. From all the 

 experiments in conjunction, it appears that the numbers of 

 vibrations performed in a mean solar day at Paris (reduced 

 as usual) by the two pendulums, were respectively 85922*06 

 and 85933-83. 



The pendulums and apparatus were reconveyed to London, 

 early in September, by water ; and the rates again determined 

 at Mr. Browne's house in Portland-place, by means of that 

 gentleman's excellent clocks and transit observations made by 

 Capt. Sabine. The precautions used are fully detailed ; and 

 the observations, which are also appended in a tabular form, 

 the author being assisted by M. Quetelet, of Brussels. They 

 give as a final result 85933*29 and 85945*85 for the numbers 

 of vibrations respectively made by each in a mean solar day, 

 similarly reduced for London. 



As a final result of the whole operation, the author regards 



" 12 s *00 



