497 
MAGNETIC OBSERVATIONS, 
DISCUSSED BY 
MAJOR SABINE, R.A.,F.R.S. 
1. Observations of the Dip. 
Captain Fitz-Roy was furnished with two Dip Circles, one by 
Gambey, and the other by Dollond ; the latter supplied by Govern- 
ment, and Gambey’s purchased by himself. : 
Gambey’s, being found to give results more accordant with 
each other than Dollond’s, was used at all the stations, except Rio 
de Janeiro. The Circle was nine and a-half inches in diameter, 
and was furnished with two needles. This instrument was, in all 
respects, a very superior one. It was placed for observation on a 
stand, which raised it from two to three feet above the ground. 
The needle was observed in eight positions, and as the readings 
accorded sufficiently well with each other, their arithmetical mean 
has been taken as the dip resulting from the observation. The 
eight positions were as follows: 1, with the graduated face of the 
circle towards the east; 2, with the same towards the west. The 
needle was then taken out and replaced with the ends of the axle 
changed, so that each end rested on a different plane to what it 
did before ; it was then observed, 3, with the face of the circle 
towards the west, and 4, with the same to the east. The poles 
were then inverted, so that the end of the needle which was before 
a north pole became a south pole, and the four positions were again 
repeated. The arc indicated by both ends of the needle was read 
in every position: an observation of the dip consisted, conse- 
quently, of sixteen readings. In the subjoined tabular record these 
are comprised in four entries, a mean being taken of the arc read 
at the two extremities of the needle, and of the positions 1 and 
3, 2 and 4: 1 and 3 form the column, a; 2 and 4 the column, a’; 
and the same positions, with the poles reversed, the columns 
a” and a”. 
VOL, I. 2k 
