54)6 MAGNETICAL OBSERVATIONS. [aPP. N° IX. 



5. Tlie anomaly of variation bears a certain proportion to 

 the dip of the needle, being greatest where the dip is great- 

 est, diminishing as the dip decreases, and disappearing alto- 

 gether on the magnetic equator. 



a. Captain Flinders ascertained, that the medium error or 



anomaly for 8 points deviation of the Investigator's 

 head, on either side of the magnetic meridian, was 

 very nearly ^^^th of the dip, .05 the decimal expression 

 of which, he considered to be the common multiplier to 

 the dip, for obtaining the radius of error at any si- 

 tuation in the southern hemisphere ; and .053 to be the 

 common multiplier, from England to the magnetic 

 equator. 



b. This, however, can only be correct wthin certain limits, 



as on the magnetic pole, where the anomaly would 

 probably be equal to the dip, or 90°, the decimal 

 multiplier would require to be increased to 1.0. Hence 

 it was suggested, by an officer in the expedition un- 

 der Captain Ross, that in those parts of the globe 

 where the dip is 90*, the compass needle would pro- 

 bably always stand N. and S., by the attraction of 

 the ship. This position clearly follows from the infe- 

 rence above, provided the compass be placed near the 

 ship's stern in mid-ships ; but if placed as described in 

 inference No. 8. the ship's head by the compass on the 

 starboard side of the main deck, would always appear 

 to be east, and on the larboard side west. 

 (c.) This last remark is further established, by the increasing 

 power of the local attraction in high latitudes, and 

 the diminishing energy of the compass needle. For 

 the directive power on a horizontal magnetic needle, 

 supposing the magnetic force to be always equal, is 

 proportional to the cosines of the dip. Hence, if the 

 directive force on the magnetic equator be called 1.0, 

 under a dip of 60°, it will be equal to i ; under a dip 

 of 70°.32', to 1 ; under a dip of 75°.31', to i ; under 

 a dip of 78°.28', to } ; under a dip of 80°.24', to h ; 



