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THE POPULAR SCIENCE MONTHLY. 



ism also reverses, so that the upper and lower ends are still as 

 tliey 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 neu- 

 tral line takes a general fore-and-aft direction. Continuing to 

 change course to the southward, the poles and neutral line con- 

 tinue 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 con- 

 ditions 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 pro- 

 cess of construction, it has been made as permanent and well de- 

 fined 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 



