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Page:Popular Science Monthly Volume 34.djvu/315

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THE GUIDING-NEEDLE ON AN IRON SHIP.

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 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 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 magnetic 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 improbable influences upon the compass, it were much more reasonable to ascribe it to the changed conditions of her magnetism by induction 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 influences were when the ship was last swung to determine the deviations 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 forward 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-