67 



NOTES TO BOOK XII. 



(c.) p. 62. THIS proposed arrangement was not suc- 

 cessful, because as the ship turns into different positions, it 

 may be considered as revolving round a vertical axis ; and 

 as this does not coincide with the magnetic axis, the rela- 

 tive magnetic position of the disturbing parts of the ship 

 and of the correcting plate will be altered, so that they 

 will not continue to counteract each other. In high mag- 

 netic latitudes the correcting plate was used with success. 



But when iron ships became common, a correction of 

 the effect of the iron upon the ship's compass in the gene- 

 ral case became necessary. Mr. Airy devised the means 

 of making this correction. By placing a magnet and a 

 mass of iron in certain positions relative to the compass, 

 the effect of the rest of the iron in the ship is completely 

 counteracted in all positions. See Phil. Trans. 1836. 



(D.) p. 63. I have stated in the text, as I had 

 written in the first edition, that the facts which have been 

 collected in terrestrial magnetism appear to await the hand 

 of the theorist. But the subject has already been taken up 

 by a theorist (M. Gauss), not inferior to any of the great 

 mathematicians who completed the theory of gravitation; 

 and institutions have been established for extending the 

 collection of the facts pertaining to it, on a scale which ele- 

 vates Magnetism into a companionship with Astronomy. M. 

 Hansteen's Magnetismus der Erde was published in 1819. 

 His conclusions respecting the position of the four mag- 

 netic " poles" excited so much interest in his own country, 



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