294 THE POPULAR SCIENCE MONTHLY. 



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 sus- 

 pended 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 ver- 

 tical 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 hori- 

 zontal, 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 con- 

 stantly 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 aX 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 hori- 

 zontal 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. 



