IV. COMPASSES. 777 



3145e. IV. Large size (lO-inch card) suitable for stan- 

 dard compass when quadrantal error does not exceed 5. 



In the improvements here illustrated the object primarily aimed at was to 

 obtain a compass to which the Astronomer Royal's correctors could be 

 applied with safety and convenience. The quadrantal correctors must not be 

 so near the needles of the compass as to sensibly affect its direction through 

 magnetization of the soft iron by the influence of the needles, otherwise the 

 quadrantal error will, if truly corrected in middle latitudes, be over corrected 

 in high magnetic latitudes, and under corrected in low magnetic latitudes. 

 Thus when the 12-inch iron cylinders of the Liverpool Compass Committee 

 are applied with their ends at a distance of 7 inches from the centre of an 

 Admiralty compass card in this country, they correct a quadrantal error of 

 12^, but of this 7| is due to magnetization of the iron by the compass 

 needles, and only the remainder or 5 is genuine, that is to say, dependant on 

 the magnetization of the correctors by the terrestrial magnetic force. Blocks 

 of iron weighing many tons would be necessary to safely correct for all lati- 

 tudes a quadrantal error of even so moderate an amount as 5 or 6, when the 

 compass needles are of so great magnetic moment as those of the Admiralty 

 standard compass. But if, as in the several sizes of compass now exhibited, 

 the needles are of thin steel wire from an inch and three quarters to three 

 inches long, a quadrantal error of any amount not exceeding 21 may be cor- 

 rected perfectly in all latitudes by a couple of globes of iron of not more than 

 6 inches diameter fixed on two sides of the compass. 



To correct a quadrantal error of 21| a couple of globes, each 6 inches in 

 diameter, fixed on the two skies of the binnacle at a distance of 6 inches 

 asunder may be used, with the, bearing point of the compass midway between 

 them. Hence, to allow room for the case containing the compass, and for 

 the gimbals supporting it, the diameter of the compass card must not be more 

 than 4 inches (I.). This is the smallest of the four sizes now exhibited. 



When the quadrantal error is 11 the globes, if of 6 inches diameter, must 

 be placed 9 inches asunder and the 6 inches diameter compass card (II.) may 

 be used. If with the same size of globes the distance asunder is 12 inches 

 the quadrantal error corrected is 6, and, therefore, when the quadrantal 

 error is of this amount or anything less the 8-inch card (III.) may be 

 used. 



The binnacle exhibited is suitable for 4-inch or 6-inch compass cards. It 

 contains two adjustable magnetic correctors for the semicircular error, one for 

 neutralizing the athwart ship component, the other the fore and aft component 

 of the ship's magnetic force. The athwart ship corrector suffices to correct an 

 error of about 23, whether to port or to starboard, when the ship's head is 

 north or south ; the fore and aft corrector corrects an error of like amount 

 when the ship's head is east or west. The binnacle also contains, in the four 

 edges of its square top, provision for placing securely a pair of bar magnets 

 athwart ship and another pair fore and aft in convenient positions a little 

 below the level of the compass card ; the former pair to be used when the ath- 

 wart ship component of the ship's magnetic force, the latter when its fore and 

 aft component exceeds the amount neutralizable by one or other of the 

 adjustable correctors. Thus the binnacle with its several appliances now ex- 

 hibited supplies convenient means for thoroughly carrying out the complete 

 system of compass correction set forth by the Astronomer Royal in his paper 

 on the correction of the compass, published in the Transactions of the Royal 

 Society for 1839, according to the following very simple rule in three parts, 

 as follows : 



I. Place the ship's head north or south magnetic and bring the compass to 

 point correctly by the athwart ship correcting magnets. 



