82 Fisher on the Variation of the Compass. 



The Line of no Deviation, is a term lately applied to obser- 

 vations of this kind, signifying that point of the compass into 

 which, if the ship be placed, the direction of the needle will not 

 be affected by the iron in the ship ; that is, if we conceive the 

 whole quantity of iron in the ship to form one magnet, it is that 

 point of the compass in which, if she be placed, the axis of this 

 magnet coincides with the magnetic meridian. 



By the rule given by Captain Flinders, it will appear that the 

 error is a minimum when the direction of the ship coincides with 

 the magnetic meridian, therefore the line of no deviation must 

 coincide with the magnetic meridian ; if this were the case, and 

 the magnetic dip and force constant, then, indeed, this correction 

 would apply, since that part of the disturbing force which is 

 wholly efficacious, would, by the resolution of forces, be as the 

 sine of the angle contained between the magnetic meridian and the 

 ship's axis. It is natural to conceive that if the iron in a ship 

 were equally distributed, the magnetic axis of the ship would 

 nearly coincide with the ship's axis, or at least be in the same 

 plane. However this may be, it was far from being the case 

 either in the Dorothea or the Trent, particularly in the first, for 

 in no position of her head butW^S., or the opposite point E|N., 

 would the needles in the ship point in the direction of the mag- 

 netic meridian ; consequently the magnetic axis of the ship made 

 an angle of no less than about 67° with the ship's axis. This 

 was confirmed several times during the voyage, particularly on 

 the 23d and 24th July ; the ships were then closely beset many 

 miles with ice, in lat. 80°.17' N., and from its rapid motion 

 by means of currents they were turned in every direction; this 

 afforded an opportunity of comparing the binnacle compasses 

 with a compass fixed on the main-top-gallant-mast head, judi- 

 ciously placed there by Captain Buchan, for the purpose of ob- 

 taining magnetic bearings, free from the local attraction of the 

 ship. On comparing these compasses, it was found that the 

 only position in which they would agree was when the ship was 

 in the direction W^S, or the opposite point ElN ; but when the 

 ship's head was SW^S, by the mast-head compass, the deck 

 compasses where W. by SiS., being a difference of about 31°. 



