164 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
iron on either side of the principal axis of the ship continued as at Gillingham, the 
observations showing only those very small differences in the exact points of no dis- 
turbance, which have been remarked in the Erebus, and which we may be content 
to view as accidental differences. From these observations, pursuing the usual course, 
we obtain b — - j- ’9873, and a = + -0292. Here also, as in the case of the Erebus, the 
observations on the first arrival at Hobarton give a somewhat less value for a tan 6 
than those at Gillingham. It is possible that a similar series of experiments may 
have been repeated in the Terror on the return to the same station in 1841, but no 
record of it has been received in England, and the observations of 1840 are expressly 
referred to, in a note appended to them, as furnishing the corrections for the declina- 
tions observed between the months of October 1840 and April 1841. I think it not 
improbable that if the ship were swung in 1841, the resulting value of a tan & will 
prove, as in the Erebus, to be somewhat greater than in 1840, and that the mean 
value of a at Hobarton will thereby come into a closer accord with its value at Gil- 
lingham. 
The practical effect of so small a difference is however unimportant, and I have 
taken a in round numbers for the declinations under consideration = + *030, and 
l = + '9875, and have computed with these values the Tables from which the correc- 
tions for the Terror’s declinations have been taken. 
Part of the materials required for the correction of the observations of inclination 
and intensity, made in the Terror during the voyage under notice, not having yet 
reached England, the deduction of the constants c, d , and A' for that ship has been 
postponed. 
Index Correction of Needle R. F. 4 for the Observations of the Inclination in the 
Erebus. — The observations of the inclination at sea on board the “ Erebus,” were 
made with Mr. Fox’s apparatus for determining the magnetic inclination and inten- 
sity, and one needle, R. F. 4, was used throughout the observations which are now 
under consideration. The poles were not reversed ; the circle was used with the 
face east only, and the needle with its marked side towards the observer. An index 
correction is therefore required for all the sea observations, and must be sought by 
comparing the inclination shown by the same circle and needle when observed with 
in the same manner on shore, at stations where the inclination was otherwise deter- 
mined in an independent and complete manner, viz. by needles of which the poles 
were reversed, and the needle and circle used in the eight ordinary positions. 
The determinations of this description made by the Expedition at the Magnetic 
Observatory at Van Diemen Island in 1840 and 1841, at Auckland Island in No- 
vember and December 1840, at Campbell’s Island in December 1840, and on the ice 
in lat. — 68° 28', long. 176° 32', on the 8th of January 1841, — furnishing the required 
comparison, — were as follows : — 
