270 THE COMPASS IN MODERN NA YIGA TION 



that of south polal•it3^ As she gradually changes course to the 

 eastward, so will the north focus shift to the port bow, the south 

 focus to tlie starboard quarter, and the neutral line dividing them, 

 whicli while the ship lieaded north was athwartsbip, will now 

 become a diagonal from starboard bow to port quarter. When 

 the ship heads east all the starboard side is pervaded with south 

 polarit\% the port with north, and the neutral line takes a gen- 

 eral fore-and-aft direction. Continuing to change course to the 

 southward, the poles and neutral line continue their motion in 

 the opposite direction, until at the south the conditions at noi'th 

 are repeated, but this time it is the stern that is a north pole, 

 while the bow is a south pole. At west the conditions at east 

 prevail, only that it is now the starboard side that has north 

 polarity and the port side south polarit3^ And this transient 

 induction in both the cylinder and the ideal ship is sol el 3^ due 

 to the effect of the earth's magnetic field in which the3^ move. 



Leaving now the ideal or " soft " iron ship and passing to the 

 consideration of the actual ship, which is built of many beams 

 and frames that have been bent, hammered, and twisted in fash- 

 ioning them for the construction, we find that the structure, al- 

 though still containing many "soft" iron pieces that become mag- 

 nets when lying in the magnetic meridian and lose their magnetic 

 qualities when turned at right angles to that plane, has acquired 

 characteristics that make it as permanent and well defined a 

 magnet as the steel bar, with poles and neutral line as in the 

 bar, but located according to the direction, with reference to the 

 magnetic meridian, in which the ship's keel lay during the 

 course of her construction. 



An iron ship, with her frames, plating, decks, beams, stanch- 

 ions, shafts, engines, smoke-pipes, yards, and masts, is not a sim- 

 ple magnet like a steel bar, but a network of magnets having the 

 characteristics of a simple magnet growing out of many and di- 

 verse and reactionary influences within the hull. However 

 complex the network of magnets may be, yet, for purposes of 

 analytical investigation to reach results to enable the mariner 

 to allow for the influence of the ship's magnetism upon the 

 compass, its effect may be considered as taking place in three 

 coordinate axes, namely, fore-and-aft, athwartship, and verticall3^ 

 downward, with the pivot of the compass needle as the origin- 



Almost all the structural iron of a ship is symmetrically ar- 

 raiiged with reference to the vertical plane through the keel, so 

 that for any piece on the starboard side another is generally 



