TERRESTRIAL MAGNETISM.—MURPHY. 307 
tendency; besides, once that induced tendency 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 per- 
manent magnetism, hammered into it when building, it must 
adapt itself to the greater power, and thus it is the resultant 
of both has been always found, and not the individuality of 
either. 
“Time is a chief element in the acquisition and efficiency 
of this induced magnetism; for the longer a ship steers on a 
given course, or lies in the same general direction, the greater 
will be the magnetic charge, and the more slowly will it 
move and shift with the changing courses of the vessel. 
“This induced magnetism has been dwelt upon at some 
length because of its prime importance to navigation. 
“The other magnetic qualities of a ship are comparatively 
stable, but this is treacherous and changeable to a degree that 
necessitates constant vigilance to prevent disaster. On the great 
fleet of transatlantic steamers it is more likely to lead into dan- 
ger than on the other routes; the ship steers a generally easterly 
or westerly course going to Europe and returning to America ; 
the magnetic influence on the outward trip is the opposite to that 
returning; the ship runs at a high rate of speed, and the induc- 
tion varies on different parts of the route according to the inten- 
sity of the magnetic field passed over, the smoothness or rough- 
ness of the sea which affects the motion of the ship, and the 
warmth or coolness of the weather.” 
“Instead of attributing the loss of vessels when approaching a 
coast,” says Lieutenant-Commander Lyons, “to the magnetic 
effects of fog and land, and other improbable influences on the com- 
pass, it were much more reasonable to ascribe it to the changed 
conditions of her magnetism by induction during the passage, 
which has not been discovered or kept account of by frequent 
azimuths previous to closing in with the land. Suddenly a course 
the captain thought perfectly safe carries the ship upon a shoal 
or rock, and the fault is ]Jaid upon the compasses, whereas they 
but obeyed the magnetic influences that became altered, during a 
long passage, from what these influences were when the ship was 
last swung to determine the deviations of her compasses. 
“The means taken for discovering the permanent magnetic 
character a ship has acquired in building is a dry dock survey. 
Let us suppose the ship and dry dock to be parallel to the mag- 
netic meridian. Stations are established at the prominent points 
on the steps or on the side of the dock. A compass is taken 
