Page:Popular Science Monthly Volume 34.djvu/314

300 also reverses, so that the upper and lower ends are still as they were before—a south and a north pole respectively.

Hold it horizontally in the meridian, and the end toward the north becomes a north pole, while that toward the south becomes a south pole. Revolve it slowly or rapidly in azimuth, and the foci of magnetic polarity also move with the fidelity of a shadow, until, when the cylinder points east and west, all the side facing the north is pervaded by north magnetism, and all facing the south by south magnetism. Again: let us conceive the hull of a ship to be like our cylinder of metallically pure wrought-iron, and as susceptible of magnetic induction in its ever-changing courses as the cylinder is when turned round. Then, as the ship steers north (in this latitude), the bow will become the center of north polarity, and the stern that of south polarity. As she gradually changes course to the eastward, so will the north focus shift to the port bow, the south focus to the starboard quarter, and the neutral line dividing them, which while the ship headed north was athwartship, will now become a diagonal from starboard bow to port quarter. When the ship heads east, all the starboard side is pervaded with south polarity, the port with north, and the neutral line takes a general 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 south the conditions at north 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 polarity. And this transitory induction in both the cylinder and the ideal ship is solely due to the mild effect of the earth's magnetic field in which they move.

Now, to consider it in connection with an actual ship. The hull of no vessel is metallically pure, nor has it acquired shape and stability without much hammering; moreover, it can not be made an abstraction from a magnetic state. By hammering in the process of construction, it has been made 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 magnetic direction in which the ship lay on the stocks, in strict conformity to the places they occupied in the ideal vessel just described. Therefore, it is not as susceptible of the mild magnetic induction of the earth as the cylinder and ideal hull, although the straining while on a passage and the buffeting of the waves do assist the inducing tendency; besides, once that the induced magnetism becomes lodged, it does not move and shift with the freedom and facility that it did in the cylinder; and finally, as it already finds a tenacious occupant of the vessel in its permanent magnetism, hammered into it while