3o8 THE POPULAR SCIENCE MONTHLY. 



ence in every direction to wliicli that extends. Soft iron, and "both, 

 horizontal and vertical induction, are T's characteristics. 



In all cases of iron which becomes magnetic through the mild 

 inductive influence of terrestrial magnetism, it should be remem- 

 bered that this influence may be variously modified, if, indeed, not 

 in some instances entirely superseded, by the inductive action of a 

 powerful surrounding field of permanent magnetism in the hull 

 itself. 



According to the location of the bulk of each class of iron — 

 the hard and the soft, the vertical, longitudinal, transverse, and 

 unsymmetrical — its resultant or representative, which we may 

 designate as a rod or a bar, will occupy a position relative to the 

 compass, either forward or abaft, to starboard or to port ; only 

 one such position for each is shown in Fig. 16 ; there are, however, 

 two possible positions for every rod, and four for some. 



The problem has now been stated, so we will pursue it no fur- 

 ther, as the vein of solution would introduce trigonometrical for- 

 mulae. 



By swinging a ship at compass-buoys, or steaming in a circle 

 on the open sea, the magnetic effect of the ship — that is, of the 

 three imaginary magnets in the axis of X, Y, Z — is brought to bear 

 at every point on the needle, causing it to deflect from the mag- 

 netic meridian by different angles at different points. These vari- 

 ous deflections, being serially arranged, constitute what is known 

 as " a table of deviations." Upon analyzing this, the numerical 

 strength of each imaginary magnet is obtained, and further disin- 

 tegration exposes to view their individual component parts. And 

 thus it is that from effect we seek backward to an intelligent 

 comprehension of the cause. 



But as a ship sails the ocean she passes through ever- varying 

 fields of terrestrial magnetism ; also, her own magnetism is under- 

 going constant change, due to the wrenching and straining, the 

 shock of waves, and the vibrations set up by firing her battery ; 

 from this mutability of cause naturally results a variety in the 

 effect — the deviations. They are never the same. 



Let a ship proceed to Havana, and she will find them different 

 from the series determined at New York ; at Hong-Kong they dif- 

 fer from those at Rio de Janeiro ; in tropical seas they are moder- 

 ate, in polar regions enormous ; when a ship is upright, they have 

 one value ; when she heels, they have another. Their varying 

 phases are a manifestation of the strife and su.ccessive domination 

 of the three magnets whose intimate relationship has been pointed 

 out ; now it is the ship, as when she steers a certain course for 

 many days and thus strengthens her forces ; again, it is the earth, 

 when the compass ventures into her frozen strongholds, where it 

 but wavers sluggishly and totters about every course ; and finally 



