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just as the retentive magnetism is liable to change in bars or plates 

 of iron if hammered, vibrated, or bent whilst held in new po- 

 sitions. 



And it was further inferred, that the analogy with plates and bars 

 might be expected to hold, notwithstanding the numerous pieces of 

 which the ship's hull might be composed, because, in experiments 

 on combining short magnets into long series, or submitting piles of 

 short bars of iron to terrestrial induction, it was found that no ma- 

 terial difference in the effects occurred betwixt a single steel magnet 

 of a given length, or a single bar of iron, and the same substance 

 and dimensions combined in short or small pieces in contact. Hence 

 it was considered that an iron ship should exhibit in its general 

 fabric the characteristic phenomena of one undivided mass, or a 

 unity as a magnetic body. 



These anticipations, it will be seen (published betwixt three and 

 four years ago), have, so far as we have now time to elucidate them, 

 had verifications, in actual experiments on iron ships, as beautiful as 

 they are conclusive. 



In the case of the 'Elba,' an iron ship of 200 feet in length, built 

 recently on the Tyne, the magnetic condition before launching, 

 which I was invited to investigate by Mr. Robert Newall, the owner, 

 was found satisfactorily accordant with theory. Her head pointed 

 south a few degrees westerly, and her lines of no-deviation (indica- 

 tive of the position of the magnetic equatorial plane) were at a small 

 distance in elevation different on the two sides, that of the starboard 

 side being the highest. Proceeding in a direction at right angles to 

 the dip, and passing through, or near to, the ship's general centre 

 of gravity, the lines of no-attraction (descending forward) came out 

 near the junction of the stem with the keel. And there, it was re- 

 markable, as I had suggested as probable to Mr. James Napier of 

 Glasgow, before making any experiment, there was found a depart- 

 ure from the ordinary regularity of the lines of no- attraction in a 

 considerable downward bend. 



Towards the stern, the equatorial lines rose out of and came above 

 the iron plating of the top-sides, about 40 feet from the tafrail ; 

 thus giving to the after-part of the ship a uniform northern polarity. 

 The ship, consequently, had become a huge simple magnet the 

 north pole at the stern and the south at the head. The attractive 



