THE GUIDING-NEEDLE ON AN IRON SHIP. 301 



building, it must adapt itself to the greater power, and thus it is 

 the resultant of both we always find, and not the individuality of 

 either. 



Time is a chief element in the acquisition and efficacy 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 mag- 

 netic 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 trans- 

 atlantic steamers it is more likely to lead into danger than on 

 other routes : the ships steer a generally easterly course going to 

 Europe, and a westerly one coming to New York ; the magnetic 

 influence on the outward trip is the opposite of that returning ; 

 the ships run at a high rate of speed, and the induction varies on 

 different parts of the route, according to the intensity of the mag- 

 netic field passed over, the smoothness or roughness 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 to the magnetic effects of fogs and land, and other improb- 

 able influences upon the compass, it were much more reasonable 

 to ascribe it to the changed conditions of her magnetism by induc- 

 tion during the passage, and 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 laid upon 

 the compasses, whereas they but obeyed the magnetic influences 

 that became altered, during a long passage, from what these in- 

 fluences were when the ship was last swung to determine the de- 

 viations of her compasses. 



To illustrate the varied location of the poles and neutral line 

 in an iron ship while building. Figs. 9 to 12 are drawn from actual 

 cases. Imagine the ship cut in two by a vertical fore-and-aft 

 plane, and both sections opened out from aft as if turned upon a 

 hinge joining them at the bow ; the outside of each half will then 

 appear as on the paper. In Fig. 9, where the ship has been built 

 head north, the whole upper after-body is' pervaded by south 

 polarity, while the lower forward portion has north polarity. In 

 Fig. 10, where the ship was built head south, the whole upper for- 

 ward body has south magnetism, and the lower after-body north 

 magnetism — a condition of induction the opposite of Fig. 9. In 

 Fig. 11, where the ship was built head northwest, we find the gen- 



