would point to the north, at every station. But an iron ship is there: so at station 1 we find the north end of the needle repelled from the vessel; the same occurs at stations 3, 4, 16, and many others from bow to keel around the forward body of the ship. Now, only north magnetism can produce this kind of deflection: it varies in degree at each station, and where greatest there is its pole. Again: at stations 7, 8, 9, etc., we find the needle's north end attracted toward the ship; hence we have discovered the body of
Fig. 13.—Plan of a Dry-Dock with a Ship in it.
Fig. 14.—Vertical Section of Ship and Dock through the Line D D of Fig. 13.
south magnetism, for that alone can produce this phenomenon, and as with the other, so here, we locate its pole where the deflection is greatest. Finally, at stations 5, 6, etc., in an irregular path from bottom to rail we see that the needle points everywhere to the north: this is the neutral line, A sketch of each side of the ship is drawn on paper, and the degree of deflection at every station is plotted by means of the measurements from the line A, B and from the ship's side.
It has thus been shown that it is a huge magnet, the ship,
