LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
31 
Horizontal Intensity . — Captain Belcher’s determination of the horizontal intensity 
falls between those of MM. Erman and FitzRoy. 
^Erman .... 1005 
Horizontal Intensity according to< FitzRoy . . . 874 
k Belcher . . . 965 
But the differences of the three determinations far exceed in amount the errors, either 
instrumental or of observation, to which such experienced observers, provided with 
needles of steady magnetism, are liable ; neither should we be justified in ascribing 
them to fluctuations in the magnetic force itself, especially at a station where the 
dip is small, and its variations have comparatively little influence on the horizontal 
intensity. The only known cause to which we can, with any degree of probability, 
attribute them, is to station error, in an island of which the basis is a volcanic rock : 
and to the same cause we must also refer the difference of three degrees in the incli- 
nation observed by Captain Belcher at Papeite and Point Venus, stations within 
seven geographical miles of each other, with an instrument of which the probable 
error, as derived from the observations at Point Venus, does not exceed the same 
number of minutes. In such localities it is well known to observers, that disturbing 
influences, producing differences as great as and even greater than those above stated, 
frequently occur at places not many yards apart. We know the intensity observations 
at Point Venus, with Captain Belcher’s seven needles, to have been made at one spot, 
and it is worth while, therefore, to examine the degree of accordance with each other 
which their results present. Assuming the horizontal intensities determined by each 
of these needles to have an equal and independent value, and taking, therefore, the 
arithmetical mean as their most probable result, we have the errors of the needles 
as follows : — 
No. 5. Horizontal Intensity 958’6; 
No. 7- Horizontal Intensity 971 ' 8 ; 
No. 8. Horizontal Intensity 965T ; 
No. 9. Horizontal Intensity 965 T ; 
No. 11. Horizontal Intensity 966'8 ; 
No. 12. Horizontal Intensity 968‘9 ; 
No. 13. Horizontal Intensity 961 ‘4 ; 
Error — 6*8 
Error + 6'4 
Error — 0-3 
Error — 0’3 
Error + 1*4 
Error + 3*5 
Error — 4 - 0 
Mean 965’4 
Consequently the mean error g 2 * is 
/117-59 
^2 = \/ q == 4‘425, 
whence the probable error, r, of a determination with one needle is 
r — s 2 .§ V 2 = 0-674489 s 2 = 3*0, 
and the probable error of a determination with seven needles = PI. 
* Encke, Astron. Jahr. 1834, and Scientific Memoirs, vol. ii. Art. X. 
