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, ceaselessly 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 formulæ and astronomical and magnetical information that would but tire the general 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, carlings, 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 downward, with the compass-pivot as the origin. To facilitate this conception, 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, inclined to the plane of the deck, and also to that passing vertically through the keel.
As already stated, reciprocal action occurs between the magnetism 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 action 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.