152

LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM.

From (5.) and (6.), changing the signs of (6.) and summing, we have47-892 a = -4553 b + -8093 ; a = + -0263.

We have also the equations at east and west;

East .... 2-617 a = -f -0645

West .... —2-617 a — — -0753 ;

whence

“ = rii = + -° 26 7 5

or including the observations at east and west in the general sum, we havea — + -0264.

After the arrival of the Expedition at Hobarton, and before it sailed to the AntarcticCircle, a similar series of observations was made in the Erebus, on the 29th October1840, and again repeated on her return to Hobarton the following autumn, viz. onthe 29th June 1841. The south end of the needle being now the one which dippedbelow the horizon {6 being — 70° 40'), the deviation of the compass was found to takeplace in the contrary direction to that which had been observed at Gillingham, thedisturbance being towards the west as the ship’s head went round from north by eastto south, and towards the east as her head passed from south through west to north.

The line of no deviation was not found to correspond accurately with the northand south points of the compass on either of the occasions at Hobarton, but in 1840coincided more nearly with the north by west and south by east, and in 1841 withthe north by east and south by west. We may perhaps ascribe with probabilityirregularities of this nature to slight modifications in the distribution of the iron atdifferent periods, which we cannot but view as of not unlikely occurrence; for ex-ample, such as might be occasioned by the ship being secured at different times bythe starboard or the larboard chain cable. In looking through the observations ofthe Erebus, it is evident that there was no systematic or constant deviation of theplane of the ship’s attraction from that of her principal section ; but that the pointsof no disturbance were sometimes a little on the one side, and sometimes a little onthe other, of the north and south points. It appears, therefore, not improper toclass these irregularities with those others of accidental occurrence which occasionsmall discordances in partial results, and are usually ranged under the generaltechnical head of errors of observation.

If, further, we compare generally the deviations in 1840 with those of April 1841,the latter appear systematically rather the more considerable in amount. Viewed asa single fact, this circumstance might be regarcfed simply as indicating that somechange had taken place in the interim in the arrangement and distribution of theship’s iron, and an easy and natural explanation might appear to be afforded. It ishowever one of several facts which have presented themselves in the course of a care-ful examination of the observations of the first two years of Captain Ross’s expedition,which seem to point to the possibility of a somewhat different cause, viz. that when