Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

491 MAGNETIC OBSERVATIONS, DISCUSSES BV 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. Tho 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 : I , 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. 2 k