286 On the Anomaly in the Variation of the 



From these observations, and from the asssistance aflForded by 

 the lucid remarks of Captain Flinders, the inferences which fol- 

 low are deduced. 



1. Tu the constniction of every ship, a large quantity of iron 

 being used, the portions thereof which have a perpendicular po- 

 sition, such as standard and hanging knees, the nails and bolts 

 in the deck, the capstern spindle, flukes of the anchors (when at 

 sea), chain-plates, iron stanchions and riders ; the eye bolts, 

 transom bolts, joint bolts, &c. of gun carriages, and possibly the 

 upper surfaces of the guns themselves, Sjc. &c. have a tendency 

 to become magnetical, the upper ends being south poles and the 

 lower north poles, in this hemisphere, where the north end of 

 the needle dips, but the contrary in the southern hemisphere, 

 where the south end of the needle dips. 



2. The combined influence of the iron distributed through all 

 parts of the ship, seems to be concentred into a kind of magnetic 



focus of attraction, the principal south pole of which being up- 

 ward in the northern hemisphere, is probably situated, in ge- 

 neral, near the middle of the upper deck, but nearer to the stem 

 than the stern. 



Wrought iron having a much greater attraction for the mag- 

 netic needle than cast-iron, the anchors, which usually lie about 

 the bows, possess much more influence than guns ; hence, the 

 focus of attraction lies nearer to the bows than to the stern. 



3. This focus of attraction so influences the compass needle, 

 that it is subject to an anomaly, or variation from the true meri- 

 dian, different from what is observed by a compass on shore; the 

 north point of the compass being constantly drawn towards the 

 focus of attraction, which appears to be a south pole in north dip ; 

 and the south point being attracted in south dip, where the focus 

 of attraction probably becomes a north pole. 



The phaenomenon of a ship appearing to lie nearer the wind 

 when beating to the northward, with the wind at north, than 

 when beating to the southward, with a southerly wind, was ob- 

 served by my father at least 20 years ago, which phenomenon 

 Tie attributed to the " attraction of the ship upon the compass ;" 

 and ever since the year 1805, I have been in the habit of allow- 

 ing only 2 to 2i points variation on the passage outward to 

 Greenland, with a northerly or north-easterly course, but generally 

 three points variation on the homeward passage when the course 

 steered was S.W. or S.W. b. W. Without this difference of al- 

 lowance, a Greenland ship outward bound will be generally found 

 to be to the eastward of the reckoning, and homeward bound 

 will be even 4 or 5 degrees to the eastward of it. 



4. This anomaly in the variation of the compass, occasioned 

 by the attraction of the iron in the ship, is liable to change with 



every 



