MAGNETISM. ' 39 



I am of opinion, that all the experiments made by Mr, Bar- 

 low, tend to 'prove what J have advanced, although he has drawn 

 very different conclusions from me : he suggested the correcting 

 plate, to counteract the effects of the iron before and below the 

 compass. The position of the plate is ascertained, and the 

 quantity of local attraction found, by warping the ship's head 

 round the compass when she is upright, and I have no doubt 

 that if the plate be properly fixed, it will correct the course so 

 long as the ship sails on an even keel; but ships are generally 

 inclined by the force of the wind on their sails, and by the incli- 

 nation, every gun, bolt, &c., will have its polarity changed, and 

 the correcting plate will no longer correct the course. 



It has already been noticed, that long pieces of iron being 

 placed horizontally, and in the direction of the magnetic needle, 

 become polarized : this polarity is, however, very weak. Since 

 iron is actually magnetical by induction from the earth, and each 

 separate article in a ship is liable to have its polarity changed 

 by a change in its position, it becomes a duty of the utmost im- 

 portance, for seamen to guard against the dangerous conse- 

 quences of placing articles of iron in a vertical position, near the 

 steering compasses. Large pieces of metal are frequently carried 

 coastwise, and may be stowed ath wart-ships ; iron cylinders, or 

 working beams of steam engines, being thus stowed, would have 

 their polarity changed every tack, and might occasion serious 

 accidents by acting on the compass, and cause it to indicate a 

 wrong course. I have known an iron pillar, placed in a vertical 

 position under the quarter-deck of a frigate, cause a compass to 

 deviate five points from the true bearing. When the upper end 

 of the pillar happened to be in an east and west direction with 

 the compass, a very small piece of iron placed near the needle 

 might act more powerfully upon, than all the other metals in 

 the ship. Seamen, therefore, ought to study this important part 

 of their duty, as navigators ; and, in taking in cargo's of iron, 

 whether in a rough or manufactured state, they should stow the 

 hold in such a manner as to lessen the risk arising from local 

 attraction, and prepare themselves to guard against errors in the 

 course, or the melancholy results of a shipwreck. 



SiNBAD. 



