LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
167 
The inclinations observed at the four preceding stations with needle R. F. 4, with 
the face of the circle east, and the marked side of the needle facing the observer, were 
as follows : — 
o / o / t t 
Van Diemen Island — 71 06*5 ; true inclination — 70 407 ; index correction — 25-8 
Auckland Island . . — 7 3 4T3 ; true inclination —73 10*4 ; index correction — 30*9 
Campbell Island . . —74 20*3; true inclination 7 3 51*4; index correction — 28‘9 
On ice, Jan. 8, 1841 — 84 02*9 ; true inclination — 83 36*6 ; index correction — 27'3 
Mean . . . —28*2 
An index correction of — 28' has therefore been applied in the general table to the 
mean of the observations on each day with needle R. F. 4, in order to give the true or 
correct inclination, as it would have been observed by a needle in which the complete 
process of observation had been gone through. 
Elements of Calculation of the Intensity Observations. — Of the intensity observa- 
tions made with Mr. Fox’s apparatus on board the Erebus, during the period under 
consideration, a large proportion was of the angles of deflection produced by deflect- 
ing magnets. A spare needle belonging to the apparatus was used as a deflector, and 
was fitted into a cylindrical case having screws at both ends, so that the needle could 
be applied either as “ deflector N” with its north pole opposite that division of the 
circle which the north pole of the dipping needle had previously indicated as the dip, 
— or as “deflector S” with its south pole similarly applied to the opposite division of 
the circle. 
The deflectors belonging to the apparatus, being too weak to produce sufficient de- 
flections when used separately, were employed only conjointly, and are designated as 
“deflectors N and S.” The angles of deflection varied in different localities during 
the voyage in round numbers as follows : deflector S from 50° to 45°; deflector N 
from 48° to 43° ; and N and S from 23° to 20°. 
To obtain the equivalent weight to the deflecting force of the deflectors at these 
angles, we have comparative observations of the angles produced by the deflectors 
and by weights at Hobarton, Auckland, and Campbell Islands, on the ice on the 8th 
January in lat. — 68° 28', long. 176° 32', and on five different occasions on board ship 
when the weather and other circumstances were favourable, viz. on February 8th 
and 10th, March 22nd, April 1st and 6th. These were exclusive of an attempt on 
the 1st of February, which failed on account of the ship having too much motion. 
Hobarton is necessarily the primary station of the whole series of observations 
made in this portion of the voyage, being the only station at which an independent 
determination of the intensity has been made. It is also very suitable for a base 
station, because we may expect that the absolute as well as relative intensity will be 
determined with great precision at the magnetic observatory established there, and 
will ultimately furnish a correction, should one be needed, for the provisional value 
which must for the present be employed. 
