648 SEC. 15. GEOGRAPHY. 



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, carry ing 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. 



II. Place the ship's head east or west and bring the compass to point cor- 



rectly by the fore and aft correcting magnets. 



III. Place the ship's head N.E., or S.E., or N.W., or S.W., and bring the 

 compass to point correctly by the quadrantal correctors (a pair of 

 .6-inch globes now recommended). 



The whole process may be thoroughly performed with all needful accuracy 

 for a new ship in a quarter of an hour ; though of course it will be desirable 

 to take an hour or two for verifying or perfecting the correction by testing it 

 on other points than the three on which it was first made. When the quad- 

 rantal correctors have been once accurately placed they have never again to 

 be changed for the same ship, and the same place in it (except of course in 

 the case of taking on board a cargo of iron or introducing or shifting masses 

 of iron so near the compass as to sensibly modify the quadrantal error). At 

 any time aftenvards the semicircular error (which is always liable to change 

 through changes of the ship's sub-permanent magnetism, and also through 

 change of magnetic latitude in the course of a voyage) is readily annulled by 

 placing her head north or south and using the athwart ship corrector, and 

 again east or west, and using the fore and aft corrector. In a steamer this 

 may be done at sea on any clear enough night to allow stars to be seen, or 

 day when the sun's altitude does not exceed 50. When the weather is 

 moderate enough to allow her to be steered steadily for two or three minutes 

 first on one and then on the other of the two cardinal points nearest to her 

 course, the detention at worst (that is, when the course is on one of the cardi- 

 nal points) need not exceed five minutes. 



The binnacle also contains an appliance for an adjustable magnet below the 

 compass in a line through its centre perpendicular to the deck, for correcting 

 the heeling error in iron sailing ships. 



Each magnet supplied for this purpose is in two parts, joined together by a 

 hinge, so that when out of use they cannot be placed in the box provided for 

 containing them without folding them together with unlike poles close one to 

 the other, so that wherever they may be placed in the ship, they cannot dis- 

 turb any of the compasses. A stout bar magnet brought carelessly on board 

 a ship without this precaution may be as dangerous as dynamite. 



