431 



II. "On the Magnetism of Iron Ships, and its accordance 

 with Theory, as determined externally, in recent Experi- 

 ments." By the Rev. W. SCORESBY, D.D., F.R.S., Corr. 

 Memb. Inst. France, &c. Received June 21, 1855. 



The magnetic condition of iron ships is a subject of so much im- 

 portance, practically and scientifically, that I have been induced to 

 submit to the Society a few characteristic facts (hastily indeed 

 brought together) derived from recent experiments. 



In a work in the Society's library, entitled ' Magnetical Investi- 

 gations,' it was shown, by deductions from an elaborate series of 

 experiments on plates and bars of malleable iron, that the magnetic 

 condition of iron ships should, theoretically, be conformable to the 

 direction of terrestrial induction whilst on the stocks ; and the re- 

 tentive quality, which is so highly developed by virtue of the ham- 

 mering and other mechanical action during the building, should be 

 so far fixed in the same direction, as to remain after the ships might 

 be launched, until disturbed by fresh mechanical action in new posi- 

 tions of their head or keel. In this view, taking, for instance, the 

 condition of the middle, or the main breadth section of a ship on 

 the stocks, the magnetic polar axis should assume the direction of 

 the dipping-needle (with an equatorial plane, or plane of no-attrac- 

 tion at right angles to it), passing through or proximate to the 

 centre of gravity of the iron material hi such section. Thus every 

 ship should have a characteristic magnetic distribution, primarily, 

 dependent on her position whilst on the stocks ; so that, being built 

 with the head north or south, the equatorial plane should appear 

 externally on the same horizontal level, the polar axis only being 

 inclined from the vertical, in correspondence with the direction of 

 the axis of terrestrial magnetism (Magnetical Investigations, vol. ii. 

 pp. 331, 332). 



It was also inferred, that whilst such individuality of the mag- 

 netic distribution would be rendered retentive on the same principles 

 as this quality of magnetism is developed in bars or plates of iron 

 by mechanical action, so the axial direction of the ship's magnetism 

 would be liable to change, under mechanical action, in new positions 

 of the ship's head, or under new relations of terrestrial magnetism, 



