497 



MAGNETIC OBSERVATIONS, 



DISCUSSED BY 



MAJOR SABTNE, 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 

 i-espects, 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. T. 2 K 



