1820.] Philosophical Transactions for \S\9, Part I. 47 



points of variation on the homeward pasf5age, when the course 

 steered was S.W, or S.W. by W, Without this difference of 

 allowance, a Greenland ship outward bound will be generally- 

 found to be to the eastward of her reckoning, and homeward 

 bound will be even four or five degrees to the eastward of it. 



4. This anomaly in the variation of the compass is liable to 

 change with every alteration in the dip of the needle, in the posi 

 tion of the compass, or in the direction of the ship's head. 



It would appear from Mr. Scoresby's trials, that the magnetic 

 intensity is greater in England than in Greenland ; at least the 

 vibrations of the dipping needle were quicker in England than in 

 Greenland. An oscillation performed in England in five seconds 

 took up in Greenland s x seconds. 



5. The anomaly of variation bears a certain proportion to the 

 dip of the needle, being greater where the dip is greatest, 

 decreasing as the dip decreases, and disappearing altogether on 

 the magnetic equator. 



6. A compass placed near the stern amid ships of the quarter 

 deck is subject to the greatest anomaly when the ship's course is 

 about west or east, because the focus of attraction then operates 

 at right angles to the position of the compass needle ; but the 

 anomaly disappears when the course is about north or south, 

 because the focus of attraction is then in a line with, or parallel 

 to, the compass needle, and consequently has no power to 

 deflect it from its direct position. 



7. The greatest anomaly with the compass in the position last 

 described being ascertained by observation, the error on every 

 other point of the compass may be easily calculated ; the anoma- 

 lies produced by the attraction of the iron in the ship beino- found 

 to be proportionate to the sines of the angles between the ship's 

 head and the magnetic meridian. 



8. A compass placed on either side of the ship's deck directlj!- 

 opposite to, or abreast of, the focus of attraction, gives a correct 

 indication on an east or west course ; but is subject to the 

 greatest anomaly when the ship's head is north or south. 



9. A compass placed within six or eight feet of a capstern 

 spindle, or other large mass of wrought iron, foregoes, in a great 

 measure, the influence of the focus of attraction, and submits to 

 that of the nearer body of iron. 



10. When the iron in a ship is pretty equally distributed 

 throughout both sides, so that the focus of" attraction occurs in 

 midships, a compass placed in the midship line of the deck 

 (drawn longitudinally) will be free from any anomaly from one 

 end of the ship to the other, when the course is north or south ; 

 but on every other course, anomaly will generally appear increas- 

 ing as the angle oetween the ship's head and the magnetic 

 meridian increases until the error i& at a maximum, when the 

 course is east or west. 



