ON COMPASS CORRECTION. 23 



was such that according to it a long bar of iron would have the same 

 degree of magnetisation by induction whether its length is held along 

 or across the lines of magnetic force. It seems strange that the 

 widely divergent character of the actual phenomena from those which 

 would result from this assertion did not strike him, his sole ground for 

 making the assumption being that there were no means of calculating 

 the difference between the amounts of effect in the two cases, and 

 that, therefore, they might be taken, for the sake of the investigation, 

 as being the same. This theoretical error was pointed out by Mr. 

 Archibald Smith, and an exceedingly curious result as regards the 

 effect of ships magnetism on the compass was also pointed out, when 

 the deviation from Mr. Smith's theory was also taken into account. 

 One point as regards the accuracy of the Astronomer Royal's method 

 of calculation, which would touch on this very important theoretical 

 point noticed by Mr. Archibald Smith, was that whereas, according to 

 the Astronomer Royal's theory, the compass would point correctly on 

 every course if corrected by magnets and soft iron placed in a certain 

 manner, which he points out ; according to Archibald Smith's 

 theory there might be a constant deviation left uncorrected, so that for 

 positions of the ship all round, the compass would point at a certain 

 angle from the true north and always in the same direction. This is 

 an exceedingly interesting result, and the consequences have been 

 followed out with great care and worked out by an exceedingly curious 

 and beautiful analysis, the method of doing which was pointed out by 

 Mr. Archibald Smith, which has been followed now most ably for 

 about thirty years in the Hydrographic Department of the Admiralty. 

 The nature of the disposition of iron which should produce that pecu- 

 liar effect discovered by Archibald Smith is essentially one in which 

 the masses of iron shall have a certain skewness or want of symmetry 

 in relation to the compass, so that when the compass is placed amid- 

 ships, and when the iron of the ship is symmetrical, as it usually is, 

 this peculiar term does not exist. Practically, as the compass is 

 placed in most ordinary ships, the amount of this effect is so small 

 that it may be regarded as practically insensible, and the indications 

 of it, which are found in the records of the Hydrographic Depart- 

 ment, may be considered as being in some degree due to inevitable 

 errors of observation. Still, I think I may be borne out in saying 





