306 TERRESTRIAL MAGNETISM.—MURPHY. 
instantly the upper end becomes a south and the lower a north 
pole (in this latitude). Reverse it as quickly as we may, and the 
magnet 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 towards 
the north pole becomes a south pole. Revolve it slowly or rap- 
idly 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 suscep- 
tible of inagnetic 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 centre of north 
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 the north, and the neutral line takes a general 
fore-and-aft direction. Continuing to change course to the 
southward, the poles and the neutral line continue their motion 
in the opposite direction, until at south the conditions of 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 of east prevail, 
only that it is now the starboard side that has north 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 the connection with an actual ship. 
The bull of no vessel is metallically pure, nor has it acquired 
shape and stability without much hammering; moreover, it 
cannot 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 deseribed. Therefore it is not as _ susceptible 
of mild magnetic induction of the earth as the cylinder 
or ideal hull, although the straining when on a_ passage, 
and the buffeting of the waves do assist the inducing 
