LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
161 
clination in the Erebus, having for arguments 0 and ; the corrections in the general 
Table of the inclination observations were thus obtained. 
We may compute the value of the remaining constant A f from the variations of the 
magnetic intensity observed on board the Erebus at Hobarton with Mr. Fox’s inten- 
sity apparatus, with the ship’s head on the sixteen principal points of the compass ; 
two series of such observations were made, one in October 1840 with needle R. F. 4, 
the other in July 1841 with needle R. F. 5 ; expressing the value of the intensity on 
shore by 1*82 = <p, the several values on board ship are shown in the following 
Table 
Ship’s head by 
compass. 
N. 
N.N.E. 
N.N.W. 
N.E. 
N.W. 
E.N.E. 
W.N.W. 
E. 
W. 
E.S.E. 
w.s.w. 
S.E. 
S.W. 
5.5. E. 
5.5. W. 
s. 
October 1840. 
1'792 
1-787 
1-807 
1-789 
1-803 
1-816 
1-830 
1-828 
1-832 
1-830 
1-848 
1-842 
1-862 
1-858 
1-863 
1-864 
Intensity. 
July 1841. 
1-806 
1-803 
1-818 
1-813 
1-816 
1-821 
1-832 
1-823 
1-829 
1-829 
1-837 
1-853 
1-855 
1-859 
1-857 
1-864 
Means. 
1-799 
1-795 1 
1-812/ 
1-801 1 
1-809/ 
1-818 1 
1-831 / 
1-826 1 
1-830 / 
1-8301 
1-842 / 
1-848 1 
1-858/ 
1-8591 
1-860/ 
1-864 
1-799 
1-835 
1-805 
1-8245 
1-828 
1-836 
1-853 
1-8595 
1-864 
= <?' 
= P' 
= <P' 
= <?' 
= <?' 
= <?' 
= <P' 
= <P' 
= <P' 
Employing the formula 
<fr' 
A'$ 
sin 0' — c cos 0 cos £ + d sin 6 
with the values of & and £ computed by means of the constants a, b, c and d, already 
deduced, and with the observed values of <p, 0, and <p', we have A' as follows : — 
At N. A' = 0-997 
At N.N.E. and N.N.W. A' = 0-996 
At N.E. and N.W. A' = 0-998 
At E.N.E. and W.N.W. A' = 0*993 
At E. and W. A' = 0‘998 
At E.S.E. and W.S.W. A' = T002 
At S.E. and S.W. A' = 1*000 
At S.S.E. and S.S.W. A' = 1*002 
At S. A' = 1*002 
Mean. . . . 0*999 
We obtain the same result if we employ the observed values of 0' instead of the 
