676 MAGNETISM AND THE COMPASS. 



limits, and for equalising the directive force of the needle, must only be 

 considered approximate for the latitude in which the adjustment was 

 made, and they should lose no opportunity of verifying the error, both 

 in port and at sea, as it is usually constantly changing from numerous 

 causes, as before explained, the chief amongst which are Heeling, change 

 of latitude, change of cargo, collision, after repairs, and from the ship 

 remaining with her head in one direction for a length of time, &c. 



One error very difficult to adjust is that caused by the Sub-permaneni 

 Magnetism, acquired by an iron vessel lying for a long time on one course, 

 or in dock, this error having, in some cases, amounted to 10°, after lying 

 on one course for 6 or 7 days. After steering for some time on Westerly 

 courses, you may expect Westerly error if the course be changed to North, 

 or Easterly error if it be changed to South. After steering for some time 

 on an Easterly course, you may expect Easterly error if the course be 

 changed to North, and Westerly error if it be changed to South. 



The Permanent Magnetism is also hable to change in a new iron ship, 

 and it is recommended, that after launching, she should lie with her head 

 in the opposite direction to that in which she was built. 



In some regions, such as near Cape St. Francis (Labrador), Iceland, 

 and the Isles de Los, the Compass is liable to be greatly deranged by local 

 influence. Magnetic Observations, made on the French cruiser Allier in 

 1883, when off the shores of Iceland, showed that the Local Deviation 

 sometimes amounts to as much as 20° or 30°. Its influence generally 

 ceases at 5 or 6 miles from the coast, but near Mount Sneefelds, Thorbak 

 Bay, Cape Eeikianaes, Westman Islands, and particularly on that part of 

 the East coast between Papey, Skraden, and Hvalsbak Islands, the influ- 

 ence of the land extended as far as 20 and 30 miles seaward. 



From the foregoing it will be seen, that for the purposes of computing 

 the Deviation of the steering Compasses, caused by the iron used in the 

 construction of the vessel, an Azimuth Compass is employed as a Standard, 

 placed on the mid-ship line, as far as possible from any vertical objects of 

 iron, such as masts, funnels, davits, and stanchions, and no iron of any 

 kind allowed within 6 feet of it. It should also be kept at a distance from 

 any electric lighting machinery, and double wires should always be used 

 for leads. Where the electric light is used on a ship, it is also advisable 

 to find by experiment what is the effect on the Compasses ; by day they 

 may be correct, but at night, when the dynamo is at work, the Compasses 

 may be affected to the extent of several degrees. 



It is unnecessary here to describe the process of swinging a ship for the 

 Deviation of her Compasses, but this operation must not be performed till 

 her cargo is stowed, and she is in all respects ready for sea. If the voyage 

 should lead the vessel over a large extent of latitude, such as in passing 

 from the Northern to the Southern Hemisphere, or vice versa, the original 

 Deviation Tables will beoome incorrect, and new observations will be 

 necessary. 



