672 MAGNETISM AND THE COMPASS. 



review of the examples and cases given in this report ; for although vary- 

 ing Compass disturbances exist, and the inference is irresistible that they 

 arise from the nature of the iron employed in the construction of the hull 

 of the ship, there is no doubt that, by attention to a few leading prin- 

 ciples in the building and equipment of iron ships, larger and uncertain 

 sources of error may be modified and reduced within limits both of fluctu- 

 ation and amount, which will not seriously compromise the safety of the 

 ship in the hands of an ordinarily prudent seaman. 



The points of practical import to which I would invite attention are : — 



1st. The best direction, with reference to the Magnetic Meridian, for 

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

 least Compass Disturbance. 



2nd. The best position and arrangement for a Compass, to ensure small 

 Deviations, and permanency on changes of geographic position. 



3rd. The changes to which the Compass is liable, from various causes, 

 when the foregoing conditions are fulfilled. 



1. On the Best Direction for Building an Iron SJdp. 



In those built head N.E., East, West, and N.W., strong South polarity 

 (or an attractive force on the North end of the Compass needle) obtains on 

 one side of the ship, adjoining the Compass as usually placed between the 

 middle section and the stern ; the resulting disturbance is not lessened as 

 the Compass is moved in a fore-and-aft line within these limits. 



In vessels built head S.E. and S.W,, North polarity obtains under the 

 same conditions. 



In vessels built head North or South, the conditions arise, that in the 

 former the attraction is toward the stern (the topsides in their action 

 being neutral to a Compass in the middle line of the deck), and diminishes 

 in force as the Compass is moved towards the bow. In the latter, the 

 law is reversed, and small Compass Deviations are obtained as the stern 

 is approached. 



In an iron sailing ship, built head to South, there will be an attraction of 

 the North point of the Compass to the head ; and if built head to North, a 

 like attraction to the ship's stern ; and so far there would seem to be no 

 advantage in one direction over the other. But in the first case the topsides 

 near the Compass have weak Magnetism; in the second case they are 

 strongly magnetic ; the first position seems, therefore, preferable. 



In an iron steam-ship, built head to the South, the attraction due to 

 machinery is added to that of the hull ; whereas, in one built head to the 

 North, the attractive forces of hull and machinery are, in the Northern 

 Hemisphere, antagonistic, and a position of small or no " semi-circular " 

 Deviation for the Compass may generally be obtained. In iron steam- 

 vessels engaged on the home or foreign trades in the Northern Hemisphere, 

 this direction of build is therefore to be. preferred. 



2. On the Position and Arrangement of the Compass. 



The position of the Compass, whether standard or steering, must depend, 

 as will have been observed from the foregoing conclusions, on the direction 



