1 2 Transactions. — Miscellaneous. 



present moment for its existence ; and not changing its amount, or quality, or 

 direction in regard to the ship's keel, while the ship is swung round in 

 different positions. From the slowness of its changes, this has been designated 

 by Professor Airy, " sub-permanent magnetism." 



I will now proceed experimentally to show the difference between what 

 is termed transient induced magnetism and the sub-permanent magnetism 

 above mentioned. I take this piece of bar iron, and place it in the line of the 

 magnetic meridian ; parallel to the bar, near its north end, I place this small 

 compass ; the north end of the compass needle is repelled. I now place the 

 compass similarly near the south end of the bar ; the south end of the compass 

 needle is now repelled. Reverse the bar end for end ; the result is the same — 

 the end which formerly, when pointing north, repelled the north end of the 

 compass needle, now attracts it and repels the south end, showing that, 

 although the bar has reversed its position, the magnetic current maintains the 

 same direction. This may be termed "horizontal induced magnetism." 



I now hold the bar in the line of the magnetic dip, or nearly vertical ; 

 the xipper end attracts the south end of the compass needle, the lower end of 

 the bar attracts the north end of the compass needle. Reverse the bar by 

 turning its lower end up, and the results are the same, shewing, as before, that 

 the magnetic current maintains the same direction. This is termed "vertical 

 induced magnetism." 



In the northern hemisphere the upper end of the bar would attract the 

 north end of the compass needle, and the lower the south end — just contrary 

 to the results obtained in this hemisphere. From this cause the north end of 

 a ship's compass needle is drawn to windward when a ship heels over in the 

 northern hemisphere, and the south end is drawn to windward in the 

 southern hemisphere under similar circumstances. When sailing due east 

 or west it is evident that no deviation will be caused by the vertical induced 

 magnetism of the vessel. 



I now take this small bar of steel, which has (probably in the process of 

 rolling) acquired a certain amount of magnetism. If I hold the compass to the 

 one end of the bar, the north end of the needle is attracted, no matter in what 

 position the bar may be placed. This represents what is termed the sub- 

 permanent magnetism of the ship. At any place the deviation of the compass 

 may be accurately corrected by mechanical methods ; namely, by a magnet in the 

 athwart ship direction, fixed, at a distance determined by trial, for correcting 

 the deviation when the ship's head is north or south ; by a magnet in the head 

 and stern direction, also at a distance determined by trial, for correcting the 

 deviation when the ship's head is east or west ; and by a mass of unmagnetized 

 iron (a small box of chain. is best) at the same level as the compass in the 

 athwart-ship line, or in the head and stern line, according to circumstances, 



