ON COMPASS CORRECTION. 33 



The only reason for ever using a smaller compass than this, 

 provided this works well, except for purposes of convenience, 

 would be that if the quadrantal error to be corrected is very large, we 

 may want to bring the correctors nearer than the dimensions of the 

 compass card will allow. I have therefore, constructed a smaller one 

 with an 8-inch compass card, the length of the needle being rather 

 less than 3 inches. When there is a very great quadrantal error 

 to be corrected, as in some of the heavy ironclad ships, the masses of 

 iron required to correct it would be incoveniently great, and therefore, 

 I have taken as a convenient size for such correctors a globe of iron 

 6 inches diameter which weighs 31 Ibs. That is easily handled 

 and put in position, and does not sensibly increase the cum- 

 brousness of the binnacle. But if this is not sufficient I would rather 

 diminish the size of the compass then increase the size of the globe, 

 and therefore my practical rule would be this : if you want as large 

 a compass as this, use it, unless it requires more than a 6-inch globe 

 for quadrantal corrccton. If the quadrantal error does not exceed 5 Q 

 a pair of 6-inch globes will correct it, and with a compass of this 

 size allows two such globes to be placed at the distance of 7 inches 

 on each side ; so that for any error less than 5 this sized compass is 

 perfectly convenient. When the quadrantal error is anything between 

 6 and n I take a smaller size compass, allowing the correctors to be 

 brought within 7 inches of the centre of the card, and if the error 

 exceeds 11 and does not exceed 22 Q then 6-inch correctors are stilt 

 perfectly available for a compass of this smaller size which has a 

 6-inch card. It really is not incoveniently small. It is as large as the 

 steering compass in many ships in the Navy, and therefore there is 

 not the slightest occasion for using incoveniently large correctors by 

 having a larger compass than that. If you imagine a ship with about 

 forty inches of iron sheathing such as the ships of the future will pro- 

 bably be, and with 200 tons guns, then perhaps even with this compass 

 you might want a ton or two of iron on each side, but in such a ship 

 perhaps even that would not be much thought of. It may be con- 

 sidered desirable however to have a somewhat smaller compass, and I 

 have therefore, made a miniature one. Although so small it can be 

 tossed about or rougly handled without being in the slightest degree 

 injured. I will now briefly explain the compass card. I have an. 



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