I 



APP. N° IX.] MAGNETICAL OBSERVATIONS. 547 



under a dip of 81°. 47', to i ; under a dip of 28°.49', 

 to I ; under a dip of 83°.37, to I ; under a dip of 

 84M5', to ^5 ; under a dip of 8T.8', to ^\ ; under a 

 dip of 88°.5' to 5^5 ; and under a dip of 90°, the cosine 

 being 0, the directive power of a horizontal needle mil 

 also be 0; so that the local attraction must operate al- 

 together. 

 {d.) But as the magnetic force is found to increase as we ap- 

 proach the magnetic poles, the directive power on a 

 compass needle, with an increase of dip, must diminish 

 in a somewhat less ratio than the cosines of the dip. 

 Let d be the dip, "v the number of vibrations in a cer- 

 tain interval, say a minute, of the dipping needle tra- 

 versing in the magnetic meridian, and a; the directive 

 power on a horizontal needle ; then 

 a: = cos d. v^. 



(e.) The increase of the magnetic anomaly on approaching 

 to the magnetic pole, has been cleai'ly shown by Cap- 

 tain Ross, (" Voyage to Baffin's Bay," Appendix, 

 N*' I.) By observations on the deviation of the com- 

 pass made on board of the Isabella in Shetland, where 

 the dip is 74°.21 ^', the maximum of error was — 5*^.34' 

 on an E. S. E. course, and + 5°. 46' on a W. N. W. 

 course, giving 11°. 20' of extreme difference. But in 

 Baffin's Bay, latitude 75 — 76*', where the variation of 

 the compass was between 86'^ and 96° W. and the dip 

 between84i° and 86**, the maximum of error on a course 

 E. 17° S. had increased to — 20°.30', and to + 22" 

 on a course W. 17" N. ; thus affiarding an extreme dif- 

 ference of 42°. 30'. Hence, while the multiplier to 

 the dip, on Captain Flinders' principles, was in Shet- 

 land only .076, in Baffin's Bay it had increased to 

 0.25. 



6. A compass placed near the stern, amidships of the 

 quarter-deck, is subject to the greatest anomaly or deflec- 

 tion from the magnetical meridian, when the ship's course 



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