&8 Royal Society. 



The observations made by the author at Port Bo wen, 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 Captain Foster's remaining 

 at Spitzbergen during the late northern voyage of discovery, a fa- 

 vourable opportunity was afforded him of prosecuting this inquiry. 

 Instead of making the 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 hori- 

 zontally. The relation between the simultaneous intensities of the 

 two needles could thus be ascertained, and inferences deduced rela- 

 tive 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 inten- 

 sity. 



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 005 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-twentyj 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 two hundred 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 j and an- 

 other hundred 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 observa- 

 tions ; and the second, the mean proportional intensities at every 

 hour, in each needle, deduced from the respective times of the per- 

 formance of one hundred oscillations. From a comparison of the 

 changes occurring in the two needles, it appears, that at the time 

 when an increase took place in the intensity of the dipping needle, 

 that of the horizontal needle underwent a corresponding diminution, 

 and vice versa. On comparing these results with the hypothesis of 

 a rotation of the general polarizing axis of the earth about its mean 

 position as a centre, and employing for this investigation the formula? 

 given by Mr. Barlow in his Essay on Magnetic Attractions, it is 

 found, that the radius of this circle of rotation is very nearly eight 

 minutes. The magnitude of this radius, however, will be considerably 

 influenced by the sun's declination. 



The 



