344- 



juxta-position; but always in parts of the heavens at a considerable 

 distance from each other. 



A Comparison of the Changes of Magnetic Intensity throughout the 

 Day in the Dipping and Horizontal Needles, at Treurenburgh Bay 

 in Spitsbergen. By Captain Henry Foster, R.N. F.R.S. Read 

 May 8, 1828. [Phil Trans. 1828,^. 303.] 



The observations made by the author at Port Bowen in 1 825, on 

 the diurnal changes of magnetic intensity taking place in the dip- 

 ping- and horizontal-needles, appeared to indicate a rotatory motion 

 of the polarizing axis of the earth, depending on the relative posi- 

 tion of the sun, as the cause of these changes. By Capt. Foster's 

 remaining at Spitzbergen, during the late Northern Voyage of Dis- 

 covery, a favourable opportunity was afforded him of prosecuting this 

 inquiry. Instead of making observations with a single needle, vari- 

 ously suspended, as had been done at Port Bowen, two were em- 

 ployed, the one adjusted as a dipping-needle, and the other sus- 

 pended horizontally. The relation between the simultaneous inten- 

 sities 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 Lon- 

 gitude, 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. 



The observations on the horizontal-needle were made in the fol- 

 lowing manner : after being freely suspended by a silk thread di- 

 vested of torsion, the needle was turned somewhat more than 40 out 

 of the magnetic meridian, and the oscillations counted only when the 

 arc of vibration had decreased to 40. The times of performing ten 

 oscillations were then noted successively until 200 were completed ; 

 the terminal arc and the temperature of the instrument were also 

 registered. The oscillations of the dipping-needle were taken as 

 follows : one hundred with the face of the instrument east, previous 

 to those of the horizontal-needle being observed ; and another hun- 

 dred after the latter, with the face west, a process which gives the 

 mean time of observation nearly the same for both needles. Two 

 tables are given ; the first containing a register of the observations ; 

 and the second, the mean proportional intensities at every hour in 

 each needle, deduced from the respective times of the performance of 

 100 oscillations. From a comparison of the changes occurring in 



