778 SEC. 15. GEOGBAPHY. 



- 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 afterwards the semicircular error (Avhich 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 or chain, 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 disturb any of the compasses. A stout bar magnet brought carelessly 

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



An important objection had weighed with the Compass Department of the 

 British Admiralty against the use of quadrantal correctors in the navy. It 

 was, that they would obstruct the taking of bearings of celestial or terrestrial 

 objects for the purpose of correcting the compass or of terrestrial objects for 

 the navigational use of it. On this account the mirror azimuth instrument 

 now exhibited was designed. It not only does away with that objection to 

 the application of quadrantal correctors, but it is much more convenient for 

 ordinary use at sea than the prismatic arrangement hitherto in use. It faci- 

 litates very much the taking of star azimuths by throwing (as in the camera 

 lucida) an image of the star upon the divided circle of the compass card, 

 illuminated by the ordinary binnacle lamp), or more properly speaking, on a 

 virtual image of this scale at an infinite distance as seen through a convex 

 lens. It is easy when there is not much motion in the ship to read the posi- 

 tions of the star accurately to a small fraction of the white space between two 

 dark degree divisions on which its image is seen. The focal length of the 

 convex lens is a little greater than the radius of the circle, and thus for 

 objects on the horizon or at any altitude not exceeding 30, no farther adjust- 

 ment of the azimuth appliance than just to bring the object fairly into the 

 field of view is necessary. 



The compass consists of a light aluminium boss, with a central sapphire cap 

 (by which the compass is supported on an iridium point), and a rim of 

 aluminium of from 4 to 10 inches diameter, according to the size of the 

 compass. There is an even number of holes in the rim, and half that number 



