Open main menu

Page:Popular Science Monthly Volume 34.djvu/308

This page has been proofread, but needs to be validated.

and coming to a focus at the poles; the mariner's compass is everywhere subject to their influence, and it is this influence that gives steadiness and direction to the needle.

At e, e', e", and e'" is a magnetic needle, represented as suspended at the middle by a thread from the zenith, and assuming, as it always will, a direction parallel to a line of force. At the magnetic equator (m m) this line is parallel to the horizon, and so is the needle e'"; we go north, and the line becomes bent, so the needle inclines as at e"; proceeding further, the line bends more and the needle inclines accordingly; finally, at e it is all but vertical in the vicinity of the pole. In all these cases the force or intensity of the magnetic field steadily increases from the first toward the last position of the needle, so that, if at e'" it be made to oscillate, the motion will be slow, extend over a wide sweep, and the needle will take some little time to come to rest; at e' the vibration will be quicker, the arc smaller, and the time less; while at e we will have but a few quick, jerky movements, and then a stop, as if checked by a powerful strain.

Now, a needle dipping thus at every remove from equatorial regions is of no value to guide a ship; it must always be horizontal, and this is practically obtained by placing a small sliding counterpoise on the needle to overcome the downward pull of magnetism; it is easily adjusted with every change. In this constantly horizontal direction of the needle, however, the portion of the magnetic intensity that gives it steadiness is materially changed—lessened and more diminished as we proceed from e'" to e.

Let the length A B at e'" and F B at each of the other points e", e', and e, represent the total force of the magnetic field at those places, then the portion of this intensity that is effective in horizontal planes will be represented by the length of the line A B, which is h, h,' h" at each point, and it is readily seen that these lengths are shorter and shorter. And the motion of the needle at h, h,' and h" successively will become slower, more sluggish and uncertain. Thus the seeming paradox is explained that, as we proceed from the magnetic equator toward its pole, the compass becomes less steady and reliable, while, at the same time, the total intensity of the magnetic field increases.

If a wooden ship with no metal other than copper in her frame were to sail round the globe, her compass, adjusted for dip, would experience only those magnetic phases that have already been described as peculiar to the earth—more or less steadying force and a variation of larger or smaller amount according to location; the ship herself would exert no influence whatever. But on board an iron or steel ship, with all her metal equipment and armament, the case is far otherwise; there, contention—unceasing strife—is ever active, as we shall see hereafter.