674 MAGNETISM AND THE COMPASS. 



however, one source of compass-error (that arising from the Heeling of the 

 ship) which has not been alluded to, as the ship in all the points hereto 

 reviewed is assumed to be on an even keel, and neglect of this has led to 

 the stranding of numerous vessels. 



Experiments made in ships of the Eoyal Navy tend to prove, as also 

 does the test of experience, that when the original Compass Deviations 

 are small, the errors from Heeling are generally small in proportion ; and 

 conversely, the exaggerated errors from Heeling are the consequence of 

 exaggerated errors while on an even keel. Ample elevation from the deck, 

 in order to raise the Compass above the level of the topsides and adjacent 

 deck-beams, is one of the chief conditions for reducing this source of error. 

 With head built North, on Heeling, the North end of the Compass needle 



will be attracted to the weather or nearest 

 side, from its South polarity. 

 „ N.E. ,, „ „ the same. 



„ East ,, ,, ,, the same.* 



,, S.E. ,, the North end of Che needle will have but 



little error, from the balanced conditions 



of North and South polarity of topsides. 



„ South ,, the North end of the needle will be 



repelled to the lee side, by the North 



polarity of the nearest or weather topside. 



„ S.W. ,, the North end of the needle will have 



but little error, as at S.E. 

 „ West ,, the North end of the needle will be 



attracted to the weather or nearest side. 

 „ N.W. ,, M „ the same. 



These laws only hold good so long as the topsides, in the immediate 

 vicinity of the Compass, retain their dominant polarity due to their original 

 direction of build in Great Britain. If in South Magnetic Latitudes, a 

 change of polarity takes place, the conditions of Heeling correspond to 

 such change. 



The maximum disturbance on Heeling in all these vessels is when their 

 heads are (by disturbed Compass) magnetic North or South, and this 

 disturbance vanishes when the head is East or West. This law of dis- 

 turbance may be thus explained : when the vessel's head is North or 

 South, on an even keel, (by disturbed Compass), the needle lies parallel to 

 the topsides by their combined action, which neutralize each other ; on 

 Heeling, the nearest topside exercises its then dominant polarity at right 

 angles to the direction of the needle, and hence the maximum error. With 

 the ship's head East or West, whether on an even keel or heehng, either 

 Pole ot the Compass needle points directly to the topsides, and is conse- 

 quently unaffected except in a vertical plane. 



As the amount of disturbance on Heeling varies under the various con- 

 ditions of direction of build, height of Compass, and breadth of ship or 

 distance of topsides, added to the prevailing permanent or inductive mag- 

 netic condition of the latter and the deck-beams, each ship must have an 

 individual character, to be determined only by experiment or observation 



