Mr. J. R N'apier on Ships' Compasses. 375 



" Suppose all tbe iron to be as to part perfectly hard, as to the rest 

 perfectly soft. The true result must be between the extremes ; and it is 

 quite certain, that even in an iron vessel, there is a great deal of iron 

 approaching to soft iron. We may have several cases. 



" 1. The compass may happen when placed where the deviation caused 

 by the soft iron comiwisates itself, and where, therefore, the whole de- 

 viation is caused by the permanently magnetic iron. In this case, Airy 

 will appear to be right, and the correction by magnets will answer. 

 Curiously enough, I find that this was very nearly the case with the 

 * Trident, ' the iron vessel which he selects as a test of his theory. 



" 2. Tlie compass may be placed where the permanently magnetic iron 

 compensates itself, and then the deviation would appear to be that of 

 soft iron, and would change its sign on a change of latitude, and if cor- 

 rected by magnets the error in the south hemisphere would be doubled. 

 I have not found an instance of this. 



" 3. The compass may be placed where the deviation is small, from the 

 hard and soft iron compensating each other. In this case, when the ves- 

 sel goes to the south, the two magnetisms will act in the same direction, 

 and there will be a large deviation. I think this is the explanation of 

 the deviation of the ' Bolivia' which you sent me. 



" 4. The two deviations may, in England, act in the same direction, that 

 from hard iron being the greatest. In this case, in a southern latitude, 

 the Cape for instance, the deviation will have the same direction, but be 

 diminished in amount. This was the case with one iron steamer, whose 

 deviations I got from the Admiralty, I think the ' Birkenhead. ' 



" 5. The two deviations in England may act in opposite directions, that 

 from hard iron being the greatest. In that case in the south, the de- 

 viation will remain the same in direction, but will be increased. This 

 ■was the case with another iron steamer, whose deviations I got from the 

 Admiralty, I think the ' Vulcan.' 



" Other causes may easily be imagined, and all show how uncertain any 

 corrections by magnets, or even by soft iron pillars, which I was once 

 incUned to, must be; and that the best thing to do is to keep the 

 compass as far as possible from all iron, and make frequent observa- 

 tions, &c. 



"ARCHIBALD SMITH." 



Though many captains of vessels still adhere to the principle of having 

 their compass errors corrected by magnets, the custom of placing a com- 

 pass on a mast or pole high above the vessel to be free from the influ- 

 ence of iron, as urged by Dr. Scoresby and implied by Mr. Smith, is 

 becoming more frequent. It is to be hoped this custom will soon 

 become universal. 



