156 ANNUAL OF SCIENTIFIC DISCOVERY. 



the magnetism depends cannot be expected from common tradesmen." 

 The Admiralty (I believe in accordance with precedent and with the rules 

 of the department) declined to undertake the supervision for commercial 

 ships ; and, as no other iron ships then existed, this decision amounted 

 practically to a refusal to enter on the matter. Had the subject been then 

 taken up by the government, it might perhaps have been advanced several 

 years. I did myself endeavor to collect information, and I took notes of the 

 position in which one ship was built ; but the occupations of a laborious 

 office compelled me to desist. I may mention, that in almost every instance 

 reported to me, in which the correction failed after a time, I had reason to 

 think that the failure arose from change, not in the ship, but in the correct- 

 ing magnets ; and this consideration, combined with the feeling of want of 

 leisure, prevented the extension of my inquiries. 



2. I am deeply struck with the beauty and the importance of Dr. 

 Scoresby's experiments ; and if I bring to notice the circumstance, that the 

 polar magnetism of iron plates, and the possibility of change in the magnet- 

 ism, were first strongly insisted on by myself, I trust it will not be under- 

 stood that I mean to say that those experiments are unessential to our 

 present knowledge of the subject. Still, as the first who examined into and 

 speculated upon this subject, I claim the right of criticizing the name which 

 Dr. Scoresby has proposed: and I express my opinion that " retentive" 

 (" retained" would be better in a grammatical sense) does not exactly rep- 

 resent the characteristics o the magnetis n of wrought-iron plates. The 

 latter appears to me to differ very little from the magnetism of hard steel 

 bars. A steel bar is magnetized by induction (as in an iron plate) a 

 steel bar may have its magnetism weakened or reversed : if immersed in 

 the sea- water, it would probably lose its magnetism sooner than an iron 

 ship would. But as, in practice, the magnetism of an iron ship is slightly 

 more liable to change than that of a steel magnet very carefully preserved, 

 it may be desirable that a name, expressive of that idea, should be gtven 

 to it. I would propose to call it the " sub-permanent polar magnetism of 

 wrought iron." 



3. I think it likely that the striking character of Dr. Scoresby's experi- 

 ments produces an impression of the extent of their applicability to iron 

 ships far greater than is warranted by careful consideration. We may 

 speak poetically of the shocks which a ship receives from the waves ; but, 

 in reality, the plates of iron of which a ship is composed sustain no such 

 shocks. The direct effect of the most violent sea upon them is this : that, 

 in the course of two or three seconds of time, the plate is plunged five or 

 six feet deeper in the water, and sustains the corresponding hydrostatic 

 pressure. This is very different, indeed, from the raps or slaps in Dr. 

 Scoresby's experiments, in which it is essential that the blow be of the 

 nature of impact, occupying a very small fraction of a second of time. 

 Probably the strain of extension to which the plates are subjected may 

 produce a greater effect : on this, however, experiments are wanting. But, 

 even here, the change in the state of extension is not sudden, but gradual. 



