MAGNETISM AND THE COMPASS. 673 



of the ship's build ; that is, in those built head North, the Compass must 

 be as far removed from the stern as circumstances will permit ; in those 

 built South, placed as near to the stern as convenient, without approach- 

 ing so close to the rudder-head, or iron taffrail, as to cause the ship's 

 general Magnetism to be overpowered by the magnetic influence of those 

 masses. 



In ships built East or West, there is little choice of position, except to 

 avoid, as a general rule, proximity to vertical masses of iron : in vessels 

 built with their heads on the inter-cardinal points, a position approximat- 

 ing to the bow or stern respectively, where the action from the topsides 

 (to be determined experimentally) is at a minimum, is to be preferred. 



Ample elevation above the deck, and to be strictly confined to the middle 

 line of the ship, are the primary conditions of position for every Compass 

 in an iron ship, and no Compass, whether steering or standard, should be 

 neg,rer the iron deck-beams than 4 feet ; for the steering Compass this 

 arrangement could be met by the use of a vertical card for the helmsmaa 



The Standard Compass, which, as a rule, I should recommend to be 

 invariably uncompensated, requires an elevation of at least 5 or 6 feet 

 from the deck, and to be fitted on a separate and permanent pillar or stand ; 

 it is by this superior elevation that the strong magnetic power of the iron 

 beams and adjoining topsides are correspondingly lessened. 



As every piece of iron not composing a part of, and hammered in the 

 fabrication of the hull — such as the rudder, funnel, boilers and machinery, 

 tanks, cooking galleys, fastenings of deck-houses, &c. — are all of a mag- 

 netic character differing from the hull of the ship, their proximity should be 

 avoided, and, so far as possible, the Compass should be placed so that 

 they may act as correctors of the general Magnetism of the hull. 



A Compass placed out of the middle line of the deck is affected by the 

 nearest topside, and its Deviations must necessarily be much increased if 

 that topside has the dominant polarity, as in ships built East or West. 



Experience has proved that the practical value of mast or elevated 

 Compasses has, in some cases, been overrated ; they are, in fact, affected 

 by the ship's Magnetism to an amount depending on their elevation and 

 the direction of the ship's build ; thus, in ships built North and South, 

 but especially the latter — the Compass being on the mizen-mast — the 

 Deviations will be comparatively large. In ships built East or West, the 

 Deviations will be comparatively small, from the topside, which would 

 affect a deck Compass, being more directly under the mast Compass ; they 

 may, therefore, be useful in the latter cases, and valueless in a ship built 

 head to the South. The wear and tear on the pivots and agate caps of 

 mast Compasses, from the increased motion due to their elevation, require 

 constant attention when they are employed. 



5. On various Sources of Error affecting a Comjmss placed under 



favourable Conditions 



Errors arising from changes of geographic position, as also incidental 

 causes of error due to anomalous rather than general conditions, have 

 been brought under review in the general progress of this report. There is, 



N. A. 0. 86 



