Mr. J. R Napiek on Skips' Compasses. 369 



when the Admiralty, and all the more powerful cards, would be indicat- 

 ing correctly. 



Many, if not all, compasses are said to be very unsteady at sea in heavy 

 weather. The causes of these oscillations appear to me to be correctly 

 described by Captain Walker in his " Magnetism of Ships," and also by Dr. 

 Scoresby. One cause arises from the influence of the induced magnetism 

 of the iron used in the construction of the ship, especially when the vessel 

 sails in an easterly or westerly direction and rolls heavily, for then iron 

 (the sides of an iron ship perhaps, or the iron davits of a wooden ship), 

 which at one roll or lurch becomes nearly parallel with the dip, and con- 

 sequently powerfully magnetic, attracts or repels the north end of the 

 compass needle ; by the opposite roll, may become nearly at right angles 

 to the dip, and therefore less magnetic, thereby causing the oscillations. 



The following experiments, made some years ago on a small iron boat 

 about 8 feet long by 4J broad, will serve to illustrate these views : — 



Boat's Head Magnetic. HorizontaL Inclined to Starboard. To Port. 



N +201 —20 



N.E + 151 + 241 + 3 



E + 191 + 15 4- 21 



S.B + 121 + 5 4. iQ 



S + 2 — 4 + 7 



S.W. — 8 —141 — 2 



W —15 —18 —121 



N.W —15 — 31 —231 



N — +201 —22 



Many remedies have been suggested for preventing these oscillations. 

 The most simple and effective, however, is that proposed and practised by 

 Dr. Scoresby, so long ago as the year 1822, viz., to elevate the compass 

 out of the reach of these troublesome attractions, and this method is now, 

 though for a different object, frequently adopted. 



Soft iron bars placed in the small iron boat horizontally on each or 

 either side of the compass, reduced the errors when inclined to port and 

 starboard, and it is presumable therefore would have diminished oscilla- 

 tions at sea from that cause ; but, the practice is, I conceive, objectionable 

 from the great care that I found necessary to preserve the iron from what 

 is called retentive or retained magnetism, as slight knocks entirely altered 

 the deviations. 



The other cause of the oscillations, described by Captain Walker, is in 

 the construction of the compass itself. The dip of the needle is counter- 

 acted generally, or always, in horizontal compass cards by a weight on 

 the opposite end ; and this weight by its inertia, Captain Walker says, 



