543 



It is recommended that, in any future legislation ibr the security 

 of the navigation of our mercantile marine with reference to iron- 

 built ships, the determination and record of these preliminary obser- 

 vations should be secured. 



The paper concludes by directing attention to the general prin- 

 ciples of practical import which result from the investigation, viz. as 

 to the best direction with reference to the magnetic meridian for the 

 keel and head of an iron ship to be placed in building, to ensure the 

 least compass disturbance ; the best position and arrangement for a 

 compass to ensure small deviations, and permanency on changes of 

 geographic position ; and the changes to which the compass is liable 

 from various causes on the foregoing conditions being fulfilled. 



For the best direction in building, it is shown that, from the nature 

 of the polarity of the hull, and especially of the top sides in the after 

 section of the ship and adjoining the compass, where usually placed, 

 the latter is least affected in those vessels built in the line of the mag- 

 netic meridian. 



For iron steam-vessels engaged in the home or foreign trades in 

 the northern hemisphere, it is recommended, from the then antago- 

 nistic magnetic influence of the hull and machinery, to build them 

 head to the north : for iron sailing vessels, from the top sides, in the 

 usual position of the compass, being magnetically weak if built head 

 to the south, the latter direction is to be preferred. 



The selection for the position of the compass depends on the 

 direction of the ship during building ; in those built head to north, 

 it must be removed as far from the stern as convenience will permit ; 

 in those built head to south, as near to the stern as convenient, but 

 avoiding especially, in all cases, proximity to vertical masses of iron. 

 In ships built head east or west, there is little choice of position : in 

 those built on the intercardinal points, a position approximating to 

 the stern when the action from the top-sides to be determined expe- 

 rimentally is at a minimum, is to be preferred. 



Ample elevation above the deck and exact position in the middle 

 line of the ship, are primary conditions to be observed ; and no com- 

 pass should be nearer iron deck beams than 4 feet. As every piece of 

 iron not forming a part of, or hammered in the fabrication of the 

 hull, such as the rudder, funnel, fastenings of deck houses, &c., is of 

 a magnetic character differing from the hull of the ship, proximity to 



VOL. X. 2 P 



