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Page:Popular Science Monthly Volume 34.djvu/316

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eral magnetic features of a ship built head north, only that now the north magnetism predominates on the starboard side, and south magnetism on the port side. Finally, in Fig. 12, ship's head south-west, we have the general features of the ship's head south, but with the neutral line taking a more horizontal trend, and the south

PSM V34 D316 Varied magnetic features of iron ships.jpg
Figs. 9, 10, 11, and 12.—The Varied Magnetic Features of Iron Ships, due to the Direction of their Heads while building.

polarity lessened on the starboard side and increased on the port side. And all the above is in close conformity to what theory requires.

The means taken for discovering the permanent magnetic character a ship has acquired in building, are a dock-survey, shown in Figs. 13 and 14. To simplify the explanation, let us suppose the ship and dock to lie parallel to the magnetic meridian. Stations numbered 1, 2, 3, etc., are established on the different steps of the dock, and the distance that each is from the line A, B and also from the ship's side is measured. A compass is taken successively to each station, and the direction in which its needle points is noted. Of course, if no disturbing mass were near, it