LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
121 
which sometimes for several days together raise the intensity above, or depress it 
below, its average value ; and experience shows that either from these natural causes, 
or from observation errors, or from both combined, the results obtained at the same 
station on different days with needles of assured steadiness, do occasionally vary to 
as great and greater amounts than those under consideration ; we should scarcely be 
justified, therefore, in applying any further correction. 
Employing the corrections obtained by the intercomparison of the needles, and 
combining the times of vibration of each at Panama in March 1839 with those at 
Woolwich in December 1842, we obtain the ratio of the horizontal force at Panama 
to the force at Woolwich, regarded as unity, by the several needles as follows : — 
No. 
5. 
2*078' 
7- 
2*078 
9. 
2*087 
11. 
2*082 
12. 
2*077 
13. 
2*084 _ 
Mean 2*081. 
The partial results do not differ from each other more than those might be ex- 
pected to do which should have been obtained by the repetition of observations with 
one and the same needle. 
The general table of results in the sequel has been calculated by the aid of the 
corrections thus derived, and the intensity as given by each of the needles severally 
is entered in the Table. 
The time of vibration of No. 11. at Woolwich, in October 1842, appears to have 
been affected by some accidental cause of error. Its discordance with the general 
series was perceived a few days after the observations were made, and error suspected, 
because the time of vibration would have corresponded to a considerable increase in 
the magnetism of the needle. The suspicion was confirmed by the repetition of the 
observations at Woolwich on the 27th and 28th December 1842, and at Falmouth on 
the 9th of February 1843, though no cause has been discovered either then or sub- 
sequently for the error in the first observation. Viewing the more than usual import- 
ance of accuracy in the observations at a base station, I have selected a different 
station for that purpose for this particular needle, and have chosen Singapore, both 
because the ratio of the horizontal force at that station to the force at Woolwich 
appears to have been extremely well determined by Sir Edward Belcher’s observa- 
tions in 1840 and 1841, and because any error in that determination will be corrected 
with certainty before long by the absolute determinations at the magnetic observa- 
tory there. The horizontal force at Woolwich being unity, its value at Singapore is 
2*135 by Sir Edward Belcher’s observations in 1840, and 2*140 by those in 1841. 
The mean is 2*1375. The time of vibration of No. 11. at Singapore was 468 s *7 in 
1840, and 467 s *3 in 1841 ; mean 468 s *0 at 73° Fahr. 
