[ 11 ] 



II. Contributions to Terrestrial Magnetism. — No. II. 

 By Lieut.-Colonel 'Edward Sabine, R.A. V.P.R.S. 



Received February 4, — Read February 11, 1841. 



§ 3. Captain Belcher's Observations on the west Coast of America, and the adjacent 

 Islands. § 4. New Determination of the Magnetic Elements at Otaheite. 



§ 3. Captain Belcher's Observations on the west Coast of America, and the adjacent 



Islands. 



1 HE observations, an account of which is now presented to the Society, were made 

 by Captain Edward Belcher, R.N. and the officers of H.M.S. Sulphur, employed in 

 the years 1837 to 1840 in surveying portions of the west coast of North America. 

 The account has been drawn up from the official reports transmitted to the Admi- 

 ralty, and placed in my hands by the Hydrographer, Captain Beaufort. The 

 services which Captain Belcher and his officers may be expected to render to mag- 

 netical science are not terminated, as the Sulphur has not yet returned to England : 

 but the portion now communicated forms a complete series, comprising the results 

 of their labours up to the period of their final departure from the coast of America. 

 The zeal, perseverance, and care with which these have been conducted will be best 

 appreciated by an examination of the details. 



Horizontal Intensity . — Captain Belcher joined the Sulphur at Panama in the spring 

 of 1837, receiving from his predecessor, Captain Beechey, a six-inch inclination in- 

 strument by Robinson, and several needles for experiments on the horizontal inten- 

 sity by the method of vibration. He had taken with him from England a nine-inch 

 altitude and azimuth instrument with attached needles, and a five-inch theodolite, 

 both by Gary, which he had employed in former surveys in determining declinations, 

 and had bad reason to confide in. Before his departure from Panama on a surveying 

 cruize, which might furnish opportunities of magnetic observation at several stations 

 on the west coast of America between Behring Strait and Peru, the times of vibra- 

 tion of the horizontal needles, eleven in number, were carefully observed, in March 

 1837, at a convenient spot near the ruins of the Convent of St. Francisco ; and these 

 observations were repeated at the same spot on the return of the Sulphur to Panama 

 in October 1838, after an absence of eighteen months. By comparing the times of 

 vibration in March 1837 and October 1838, as given in the subjoined Table, it will be 

 seen that the magnetism of several of the needles had greatly altered in the interim : 



c2 



