LIEUT.-GENERAL SABINE ON TEREESTKIAL MAGNETISM. 



4Yl> 



Table XVIII. — Values of the Intensity of the Force, as observed and as corrected, on 

 the sixteen principal points of the compass at Simon's Bay in April 1843. 



Index-corrections. — ^The cards of the Standard Compasses, both of the 'Erebus' and 

 ' Terror,' were unchanged during the whole of the third year's survey. The index-cor- 

 rections appear to have been very carefully watched and frequently examined. In me- 

 moranda preserved in the handvsTiting of Sir James Ross and of Captain Ceoziee, the 

 corrections are stated to have been constant from the departure of the ships from the 

 Falkland Islands in September 1842 to their arrival at Simon's Bay in April 1843. 

 The card of the ' Erebus ' is stated by Sir James Ross to have had an index-correction 

 of —1° 48' ; those of the ' Terror,' as stated by Captain Ceoziee, 



Card P=--0° 40', and Card R=+l° 13'. 



The index-corrections of the inclination-needles employed in the ' Fox' circles in the 

 sea observations (R. F. 5 in the ' Erebus,' and F. C. B. in the ' Terror') have been 

 examined by comparing their results with those of needles whose poles were reversed and 

 the inclination observed in the eight usual positions of the circle and needle, on several 

 occasions at the same identical spots either on land or on the ice. With the needles 

 of the ' Erebus' six such occasions presented themselves in the course of the three years* 

 survey (confining the comparison to land stations sufficiently free from station error). 

 They are as follows : — 



