24 L1EI!T.-C0L0NEL SABINE ON TERRESTRIAL MAGNETISM. 



At four of Captain Belcher's stations in North America, he was preceded in ob- 

 servations of the horizontal intensity by Mr. David Douglas, who visited California 

 and the Columbia River in the years 1830 to 1833. It may not be out of place to 

 examine here the degree of accordance in the results obtained by the two experi- 

 menters at the four stations. Fort Vancouver, San Francisco, Monterey, and S^* Bar- 

 bara; and the comparison will be found instructive. Mr. Douglas's observations 

 were made with two pairs of needles, which, before his departure for America, were 

 vibrated in the environs of London, at intervals of several months, with consistent 

 results. One pair of needles, numbered 3. and 4, were returned to England from 

 San Francisco in 1831 to have their magnetic state re-examined: they arrived safely, 

 and were vibrated in 1836, when, on a comparison with their rates in 1828 and 1829, 

 No. 3. was found to have slightly gained, and No. 4. to have slightly lost magnet- 

 ism ; the consequence, probably, of their having been kept in constant contact with 

 each other (No. 4. being a more powerful magnet than No. 3.), except when used 

 in observation, when both needles were always vibrated, and their combined results 

 considered as one determination. The mean of the times of vibration of these 

 needles in 1828-1829, and in 1836,, consequently furnishes a satisfactory London 

 rate for the intervening years. The second pair of needles, numbered 5. and 6, were 

 in Mr. Douglas's possession at the period of his untimely death at Owhyhee in 1834, 

 as his letters contain the notice of observations made with them at the summit of 

 Mowna Kaah, and in the crater of Kiraueah, but they have not been found amongst 

 his effects sent to England. The steadiness of this pair of needles can only be judged 

 of, therefore, by their accordance everywhere with the results of Nos. 3. and 4. 



Mr. Douglas's papers are in the Colonial Office ; an account of his magnetic ob- 

 servations, which I drew up at the request of Lord Glenelg, then Secretary of State 

 for the Colonies, was presented by His Lordship to the Royal Society, and was read 

 in May 1837, but was not printed. The results of the horizontal intensity which 

 will be now referred to, are taken from that account ; they are also immediately 

 deducible from the Table of the total intensities and dips observed by Mr. Douglas 

 in North America, published in 1838 in my memoir on the magnetic Intensity of the 

 Earth, in the Seventh Report of the British Association for the Advancement of 

 Science : they are as follows : 



Horizontal intensity : London = 1000. 



Nos. 5. and 6. Nos. 3. and 4. 



_y^ 



Fort Vancouver .... 1830 . . . 1238 



I ^ < \ 



San Francisco . . 1831 and 1833 . . . 1517 



Monterey . . .1831 and 1832 . . . 1566 



S*^ Barbara 1831 . . . 1636 



or, if we regard London and Woolwich as identical in respect to the value of the 

 horizontal intensity, and express this value by 480, which Captain Belcher's obser- 



1830 . . 1220 



1831 . . 1511 

 1831 . . 1542 

 Not observed. 



