22 



LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



In this Table each needle has been given an equal influence on the mean result, with- 

 out reference to the number of observations made with it. Where the observations 

 do not afford certain and independent evidence of the unchanged state of each of the 

 needles in respect to magnetism, weights assigned from other considerations must 

 necessarily be arbitrary and uncertain. For example, at the last station in the Table, 

 Bow Island, thirty-six observations were made with No. 5, and not more than three 

 or four with each of the other six needles. But we have already seen, on intercom- 

 parison, reason to suspect that No. 5. may have sustained a slight loss of magnetism 

 at the station preceding Bow Island, and it is the only needle in which any change 

 of the kind is indicated subsequently to the general comparison at San Bias. Whilst, 

 therefore, on the one hand, we might not be justified, without more clear and decided 

 evidence, in altogether setting aside the result with No. 5, so on the other hand we 

 should not obtain the most probable final deduction, by giving to that result a weight, 

 in comparison with that of each of the other needles, proportioned to the number of 

 observations, and resting on the probable error of a single observation, — apart from 

 changes of magnetism in the needle itself. 



We will now revert to the stations visited in the first voyage which were not sub- 

 sequently revisited, and at which the values of the horizontal intensity may be de- 

 rived by means of Nos. 5, 6, 7, or 8. Table VII. contains an abstract of the obser- 

 vations with these needles, in all of which the times of vibration were obtained in 

 -arcs commencing with 40°. The column entitled "Corrected Times," shows the 

 mean time of vibration reduced to a standard temperature of 60°. The arithmetical 

 mean of the times at Panama in March 1837, October 1839, and March 1839, has 

 been taken as the approximate time of vibration at Panama throughout the interval ; 

 and the ratio of the horizontal intensity at the other stations has been computed 

 accordingly, as shown in Table VIII. In this Table, as in Table VI., each needle 



