THE GUIDING-NEEDLE ON AN IRON SHIP. 



293 



pass-needle is tlie important factor of this article, since the needle, 

 as an instrument of navigation, is specially treated, it will be 

 necessary to dwell further on that element. It has already been 

 stated that poles of opposite name attract and those of the same 

 name repel each other. Now, on the earth, the pole nearest the 

 geographical north is commonly known as the north magnetic 

 pole, and the end of the needle pointing to it is also spoken of as 

 the north pole, whence repulsion would, of necessity, seem to re- 

 sult ; but this is an unfortunate use of terms that has grown up in 

 daily life. The real state of the case is, that whichever of the two 

 — the earth's pole, or that of the compass — we agree to designate 

 as north, the other, having magnetism of the opposite kind, must 

 be called south, and hence attraction naturally takes place. To 

 show the variability of this attraction in direction and amount in 

 various parts of the globe, we will examine Fig. 3. 



FiQ. 3.— Vaeiabilitt of the Earth's Magnetic Force. P, geographical pole ; N, magnetic pole ; 

 m m, magnetic equator ; Z, zenith ; s t, K K, df, etc., earth's lines of magnetic force. 



Let us conceive the air filled with iron particles as it is with 

 congealed vapor on a wintry night : they will not float about, list- 

 less and without form, but, like the frosty foliage on a window- 

 pane, will seem projected from a parent stem, shooting up and 

 out in graceful, wavy filaments. They are the earth's magnetic 

 lines of force permeating space (how far, I do not presume to say). 



