152 APPENDIX. 



which represents the needle at Sealiorse Point, the hori- 

 zontal directive intensity will be to the direct intensity as 

 N/i" to Pn", or as co-sine 86° 32' to radius. If, therefore, 

 the direct intensity were the sames in both cases, the hori- 

 zontal intensities would be to each other as co-sine of 70° 

 to the co-sine of SG° 32'. The intensities, however, are not 

 quite the same, but vary inversely, as a/(4 — 3sin. ^(5'), and 

 which being properly introduced, gives the law above- 

 stated ; namely, that the horizontal intensity in the two 

 places are as ^ (3-l-sec.^70°) : ^(3 + sec.^S6° 32'), or as 1 

 to 5, a reduced force, which is amply sufficient to account for 

 the general sluggishness of the needles as recorded in the 

 journal. 



But it appears that the needles were more inactive in one 

 position of the vessel than in another ; let us then examine 

 whether this is a result which is consistent with our general 

 theory. 



On this point it must be remembered, that the upper parts 

 of all iron bodies, in places of great dip, possess the same 

 species of magnetism as the pole of the earth, towards Avhich 

 the dip is made. The needle on shipboard, therefore, is under 

 the compound influence of the earth and of the iron of tlie 

 vessel, and the compass being farther aft than the great 

 body of the iron, and above it, will have its north end 

 drawn towards the vessel's head. When, therefore, the 

 head is to the southward, the magnetism of the ship will draw 

 the north end to the southward, while the magnetism of the 

 earth will draw it to the north ; so that in this position of 

 the ship, the two forces counteracting each other will de- 

 stroy the effect of either, at least when they are equal? 

 and in all cases the directive power of the needle will be 

 only that due to the difference of the two forces. Let us, 

 then, examine how nearly the magnetic power of the earth 



