12 



LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



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- 



