A comparison of the changes of magnetic intensity throughout the day in the dipping and horizontal needles, at Treurenburgh Bay in Spitsbergen

Abstract

The observations made by the author at Port Bowen in 1825, on the diurnal changes of magnetic intensity taking place in the dipping- and horizontal-needles, appeared to indicate a rotatory motion of the polarizing axis of the earth, depending on the relative position of the sun, as the cause of these changes. By Capt. Foster’s remaining at Spitzbergen, during the late Northern Voyage of Discovery, a favourable opportunity was afforded him of prosecuting this inquiry. Instead of making observations with a single needle, variously suspended, as had been done at Port Bowen, two were employed,— the one adjusted as a dipping-needle, and the other suspended horizontally. The relation between the simultaneous intensities of the two needles could thus be ascertained, and inferences deduced relative to the question whether a diurnal variation in the dip existed as one of the causes of the observed phenomena, or whether, the dip remaining constant, they were occasioned by a change in the intensity. The dipping-needle used was one belonging to the Board of Longitude, and made by Dollond. Both this and the horizontal-needle were made in the form of parallelopipedons, each 6 inches long, 0·4 broad, and 0·05 thick. The experiments were continued from the 30th of July to the 9th of August; and were so arranged, that in the course of two days an observation was made every hour in the four-and-twenty; that is, part of them in one day and another part in the other day.