308 Royal Society. 



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 

 power, as was expected, was highl}'^ energetic. At the distance of 

 50 feet, a compass on the level of the keel, at right angles to it 

 abreast of the stern, was deviated to an extent of above 10^ ; at 100 

 feet distance the ship's magnetism caused a deflection of about half 

 a point; and at 150 and even 200 feet there was a very sensible 

 disturbance ! 



In the case of the ' Fiery Cross,' built at Glasgow and launched 

 in January last (a case which I have elsewhere refeiTed to), the lines 

 of no-deviation, as taken for me by Mr. James Napier, were still 

 more rigidly in accordance with theory, — the difference of elevation 

 of the observed lines of no-deviation at the main-breadth section 

 agreeing with calculation, theoretically, to within an inch or two. 

 In the other case, that of the ' Elba,' a slight discrepancy as to the 

 comparative level of the lines of no-attraction on the two sides, might, 

 perhaps, be satisfactorily explained by the proximity and somewhat 

 disturbing influence of another iron ship (advanced only to the 

 frames or angle irons) on the port side of her. 



In the case of the ' Elizabeth Harrison,' a large iron ship built at 

 Liverpool, the first I had carefully examined, the correspondence of 

 the magnetic polar axis and equatorial plane with those of terres- 

 trial action was equally characteristic and conclusive. 



Hence we may perceive a sufficient reason for some of the pecu- 

 liar phenomena in iron ships of the compass disturbances and their 

 changes. We may see why a ship built with her head easterly or 

 westerly, and having the polar axis inclined over 18° or 20° to the 

 starboard or port side, should be particularly liable to compass 

 changes, if severely strained or struck by the sea with her head in 

 an opposite direction. We see why the compasses of the 'Tayleur' 

 should have been exposed to such a change as appears to have taken 

 place in her lamentable case. We- see in the case of the 'Ottawa*' 

 (one I have elsewhere referred to), why a heavy blow of the sea, with 

 the ship heeling and her head pointing eastward, would be likely to 

 produce a change in her magnetism, when her previous magnetic 

 distribution was solicited by terrestrial action in an angle of 30° or 40° 

 of diff'erence. And we see why the deviations of the compass in iron 

 ships should, differently from those of wooden ships, be sometimes 

 westerly and sometimes easterly in ships built and trading in home 

 latitudes ; for here, whilst in wooden ships, where the iron work is 

 in detached masses, the ship can have but little, externall}^ of the 

 character of a true magnet, and can possess but small comparative 

 diff"erences from the position of her head whilst building ; in iron 

 ships, on the converse, where the ship is rendered by percussive 

 action a powerful and, retentively, true magnet, her deviating action 

 must be expected to be different, as the polarity of the head or stern 



* Where the compass suddenly changed two points. 



