LIEUT.-GENEEAL SABINE ON TEREESTEIAL MAGNETISM. 



467 



The deviations computed with these values for the dip of —53° are shown in column 5 

 of Table II., and column 6 of the same Table shows the differences between the observed 

 and computed deviations in the dip of —53°. Employing the same constants, a Table 

 of the deviations in dips of —56°, —59°, —62°, and —65°, with interpolated values at 

 the intermediate degrees, has been computed, and has been employed in assigning the 

 " corrections for deviation" in the Table of Declinations observed on board the ' Terror' 

 between Port Louis in August 1842, and Simon's Bay in April 1843. 



(Note. — It must be borne in mind that the " corrections for the ship's attraction," 

 which appear in the Tables at the close of this and the earlier Numbers of these Contri- 

 butions have, of course, the opposite signs to the " deviations" which they are designed 

 to correct.) 



Table II. — ' Terror.' Deviations of the Declination at Port Louis and Simon's Bay. 



Inclination and Total Fobce. 



In the third year of the Antarctic Survey, as in the two preceding years, by far the 

 greater part of the determinations both of the Inclination and of the Total Force were 

 made at sea with Mr. Fox's apparatus, which is fully described (as well as the modes of 

 employing it) in the article on Terrestrial Magnetism in the 'Admiralty Manual of Scientific 

 Enquiry,' third edition, 1859, Appendix, No. 3. This apparatus was always used in the 

 one selected spot in each ship ; the face of the circle always towards the east (unless 

 expressly mentioned otherwise), and the marked side of the needle towards the observer. 

 The poles of the needle were at no time reversed, and great care was taken in mounting 

 and dismounting it to avoid injury either to the axle or to the pivots. 



The index-error occasioned by the face of the needle being always directed towards 

 the east, was examined by comparison with results obtained with needles whose poles 



