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THE POPULAR SCIENCE MONTHLY. 



that is guided around an enormous magnet, the earth, by a tiny 

 magnet, the needle. 



The near approach of one magnet to another always excites 

 contention and confusion in the field they occupy, and eventually 

 the old, old story is told — the strongest alone survives. In order 

 that the powerful ship may not paralyze its little guide, great care 

 is taken to find a suitable place for it ; and on every voyage, cease- 

 lessly and without fail, a variety of observations have to be made 

 and corrections applied to the courses indicated by the compass, 

 that this may fulfill the object of its being. To explain how this 

 is done would involve a mass of mathematical formulae and astro- 

 nomical and magnetical information that would but tire the gen- 

 eral reader, besides being out of keeping with the character of 

 this article. Let it suffice to state the problem in popular phrase ; 

 to solve it would necessitate the use of other language. 



An iron ship — frames, plating, decks, beams, stanchions, car- 

 lings, engines, smoke-pipes, yards, masts, shafts, armament in a 

 ship-of-war, and numberless other parts — is not like the steel bar, 

 a simple magnet, but a network of magnetic entanglement ; yet, 

 how complex soever this may be, for the purpose of investigation, 

 to the end that proper means may be devised for coping with it, 

 its influence may be considered as taking place in three co-ordinate 

 axes, namely, fore-and-aft, athwart-ships, and vertically down- 

 ward, with the compass-pivot as the origin. To facilitate this con- 

 ception, let us contemplate Fig. 15, and let T represent a bar of 

 iron of such quality that when held upright it becomes instantly 

 magnetic through the induction of terrestrial magnetism, and as 

 instantly has its polarity reversed upon turning it end for end ; in 

 other words, what, in investigations of this kind, is technically 

 known as soft iron. 



Let this bar, supposed to be anywhere in the interior structure 

 of the ship, take the most general position possible, namely, in- 

 clined to the plane of the deck, and also to that passing vertically 

 through the keel. 



As already stated, reciprocal action occurs between the mag- 

 netism of the bar and that of the compass-needle ; the upper end 

 of the former (in this hemisphere) attracts the north end of the 

 needle and repels its south end, while, at the same time, the lower 

 end of the bar repels the north end of the needle and attracts its 

 south end. The difference in distance, however, between the near 

 ends of the bar and needle and their remote ends, enters to such 

 extent that the influence of the remote only modifies, not equals, 

 that of the near ends ; the net result may be stated as one of ac- 

 tion between the near ends only. 



We have thus to deal with but one kind of the bar's polarity ; 

 represent its force by a line of definite length, S T for example. 



