12 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
Table I. — Comparison of Intensity Needles at Panama in March 1837, and October 1838. Com- 
mencing arcs 40°. 
Periods. 
Designation of the Needles. 
1 . 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
11. 
12. 
13. 
March 1837 
October 1838. . . . 
s 
625-1 
673-1 
600-9 
608-6 
775-7 
864-0 
s 
472-7 
475-2 
S 
512-6 
514-6 
s 
532-7 
536-8 
s 
470- 4 
471- 4 
s 
434-3 
439-5 
s 
453-9 
475-8 
s 
373-8 
403-7 
s 
375-2 
395-5 
Nos. 5. 6. 7- and 8. had each undergone a small and comparatively insignificant loss 
of force; but the changes sustained by the other needles, especially by Nos. 1. 4. 11. 
12. and 13, were too great to justify the deduction of results, either from a mean of 
the times of vibration at the two periods, or on the principle of an uniform loss cor- 
responding to equal intervals of time. Unfortunately, Nos. 1.3. and 4. were amongst 
those which had been most frequently employed at the stations visited in the cruize ; 
and as an attentive examination of the observations made with them has not furnished, 
as it sometimes does, the means of discovering when and in what manner the altera- 
tions of magnetism took place, I have not attempted to draw from these observations 
conclusions which could not be otherwise than unsatisfactory. Happily several of 
the stations were revisited in 1839, when the apparatus was in more perfect order, and 
the observers having improved by practice, the results are such as leave no other re- 
gret for the failure on the first occasion, than what is due to the loss of time and 
pains. At those stations of the first cruize which were not subsequently visited, we 
may still derive results from the observations with Nos. 5. 6. 7* and 8, which, though 
not entitled to equal confidence in respect to precision with the determinations made 
in the subsequent voyage, are nevertheless well deserving of regard and record. It 
may be convenient, however, in the relation, to invert the order of succession, and to 
commence with an account of the second, or principal magnetic, voyage. 
Having occasion to remain at Panama and its neighbourhood for some months 
after the needles had been vibrated as above noticed in October 1838, Captain Bel- 
cher repeated the observations with the needles specified in the next Table a third 
time, at the same place as before, on the 16th of March 1839. The times of vibra- 
tion inserted in this Table were on both occasions in arcs commencing with 40°, which 
had been the uniform practice with all the needles at the stations visited in the first 
voyage. Having heard from Captain Beaufort of the attention which Captain Belcher 
and his officers were giving to magnetic observations, and having been permitted to 
examine the reports of the observations of the first voyage which had reached the Ad- 
miralty on the 1st of January 1839, 1 wrote to Captain Belcher to recommend that in 
future he should commence the vibrations at an arc of 20°. This letter was received 
in Panama early in March, and a double series of observations were made in conse- 
quence on the 16th of March, one series commencing with 40° to compare with those 
of October 1838, and a second commencing with 20°, to correspond with all the ob- 
