Mr. J. R. Napiek oh Ships' Compasses. 371 



passes are adjusted. And as in sailing vessels an experimental trial 

 seldom takes place, there is no opportunity for the magnetism retained 

 while building or outfitting to be duly shaken or knocked into its natural 

 position. 



On the Clyde, on the cDUtrary, our want of conveniences for swinging 

 ships for compass deviations may have been the means of preserving our 

 iron ships from those very sudden changes, as a voyage to Gareloch 

 behind a tug steamer for two or three hours may have wrought the 

 necessary change in the retained magnetism. With our steamers the 

 effect of vibration could be more easily applied. And finding from Dr. 

 Scoresby's experiments, that a mere slap of the hand is sometimes sufficient 

 to change the polarity of a bar or plate of iron, I presume that the 

 working of our engines for three hours or more on their way to Gare- 

 loch, and making more thumping and knocking perhaps than they ought 

 to do, would be considered quite sufficient to redistribute the retained 

 magnetism, so that when the ship was swung there would be less risk 

 of any sudden change when fii'st proceeding to sea. 



The following letter from Dr. Scoresby renders Mr. Cameron's state- 

 ments about forging, hardening, and magnetizing steel bars in various 

 directions doubtful, and suggests an interesting experiment which I shall 

 now describe : — 



" Torquay, Ja7i. 3d, 1855. 



"Mr DEAR Sib, — I was uiterested by several of Mr. Cameron's statements 

 in the letter he sent me, and, had I had time, I could have explained the 

 reason (partially) why no change was known to take place in the com- 

 passes of certain ships which had been struck by the sea and returned to port. 

 The change, if it occurred, would be upset on the ship's putting about and 

 labouring by the sea with the head in a different direction. But Mr. C. 

 altogether mistakes the effects of forging, hardening, and magnetizing, in 

 the direction of the magnetic dip. I never found any (sensible) difference 

 betwixt the magnetic force of a bar or needle, whether magnetized in 

 the direction in which it had been hardened, or in the contrary. As to 

 magnetizing, it is of no consequence, the direction, if the magnet used be 

 sufficiently strong ; for a much greater power is communicated than the 

 steel can afterwards retain. 



" Pray, is the ship you have to launch shortly with her keel north and 

 south, magnetic, or nearly so ? my impression is that it will probably be 

 so. If so, an interesting and easy experiment could be made, viz., to 

 ascertain where (or at what height) on the stem and stern the ship ceases 

 to act ou the compass ? Or where, on either side of tlie bow and stern 

 (say C to 10 feet from either, towards the middle of the ship) the line of 

 attraction runs? 



