50S 



2. Observations of Intensity, 



The method employed by Captain Fitz-Roy to determine the 

 variations of the magnetic force was that of noting the time of 

 vibration of a magnetic cylinder suspended horizontally. 



The cylinder was one which had been given by M. Hansteen, 

 in 1826, to Captain Phillip Parker King, R.N., and had been used 

 by him during the survey of the coast of South America, which he 

 conducted from 1826 to 1830. The apparatus in which it was 

 vibrated, both in Captain King's and Captain Fitz-Roy 's voyages, 

 was the well-known one of M. Hansteen. 



By observations made with this cylinder on the 22d March 

 1826, and again on the 24th January 1830, in the garden of the 

 Royal Observatory at Greenwich, it appeared that its time of 

 performing 300 vibrations had increased from 734,45 seconds in 

 1826, to 775,80 seconds in 1831 ; or 41,35 seconds in 1,770 days. 

 A change of such magnitude in the magnetic intensity of the 

 instrument employed to measure the variations of the terrestrial 

 intensity, and which ought itself, therefore, to be invariable, would, 

 in the generality of cases, have prevented any satisfactory conclu- 

 sion whatsoever being drawn from the observations. Fortunately 

 from the nature of the duties in which Captain King was 

 engaged, he had occasion to return frequently to the same 

 anchorages, and by his extreme care to repeat observations on every 

 such return, he has provided a means of computing the decrease of 

 the intensity of the cylinder, proportioned to intervals of time, 

 between 1826 and 1831 ; and of thus introducing compensations 

 for it, which render the results on the whole nearly as satisfactory 

 as if the cylinder had preserved an uniform magnetic condition 

 throughout. 



The voyage which Captain Fitz-Roy had to perform promised 

 to furnish few, if any, such opportunities of examining the state 

 of the magnetism of the cylinder, between the departure from and 

 the return to England ; and, — as it cannot but be extremely dis- 

 couraging to officers to make observations which they have reason 

 to apprehend may prove unavailing from defect in the instrument 

 employed, — it must be regarded as exceedingly creditable to 

 Captain Fitz-Roy and his officers, that, with the knowledge of the 



