24 
LIEUT.-COLONEL 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 ta 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. 
r A ~ > 
Fort Vancouver .... 1830 . . . 1238 ; 
San Francisco . . 1831 and 1833 . . . 1517 ; 
Monterey . . . 1831 and 1832 . . . 1566 ; 
S ta Barbara 1831 ... 1636; 
Nos. 3. and 4. 
r , 
1830 . . 1220 
1831 . . 1511 
1831 . . 1542 
Not observed. 
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- 
