288 PROCEEDINGS OF SECTION A. 



something about the needle pointing true to the pole. This, I 

 admit, is an extreme case, but very few iron ship's compasses are 

 less than four or even five points out on some courses. 



I need scarcely mention that vessels are not allowed to proceed 

 to sea with their compasses in this condition. The Board of Trade 

 insists on their being compensated, within manageable limits, by 

 means of magnets suitably placed; and the ability to properly and 

 intelligently manipulate these correcting magnets constitutes a 

 ■very important element in the qualification of a shipmaster or 

 officer. But it is almost impossible to so adjust a compass that it 

 will be correct under all circumstances ; the varying influence of 

 the earth's magnetism, the heeling of the vessel, shifting of masses 

 of iron on board, and various other causes, combine to interfere 

 with its correct action, and the only safeguards are constant watch- 

 fulness and frequent correction. 



As an instance of the numerous and unexpected dangers which 

 threaten the compass, I may mention the case of a vessel which 

 came under my notice. The iron mainyard of this vessel was 

 simply an enormous magnet, and according to whether the port or 

 starboard yardarm was nearer the standard compass, distant 

 perhaps 60ft., the north pole of the needle was attracted or 

 repelled about half a point, making a total error on swinging the 

 yard of a full point. 



The usual method of ascertaining the error of a compass is by 

 comparison of the true bearing of an object with its bearing by 

 compass, and the difference between these two bearings will be the 

 error of the compass for that position of the ship's head. In port 

 or when near the land it is generally possible to work by the true 

 bearing of a fixed object on shore, such as a chimney, tower, flag- 

 staff, &.C., and this is the method adopted by professional adjusters. 

 At sea, however, with no land in sight, the only available method 

 is by comparison of the true bearing of a celestial body with its 

 bearing by compass, and the object of my diagram is to facilitate 

 the computation of the true bearing of such bodies as are generally- 

 used for this purpose. 



In all well-regulated ships the sun's bearing by compass is 

 noted every time that he can be observed rising or setting, and 

 this, compared with his true amplitude by calculation, gives a very 

 handy and correct method of ascertaining the error of the com- 

 pass. Few parts of the world are, however, blessed with such a 

 clear atmosphere as we have in South Australia, and in many 

 I^laces it is seldom that the exact moment of the sun's rising or 

 setting can be observed. It is also very often desirable to ascertain 

 the error of the compass at other times than when the sun is on 

 the horizon, and during the day the only available means is by 

 com])arison of the sun's true azimuth with his bearing by compass. 



I may here say, though in future I will only mention the sun, 

 that all statements apply equally to any celestial body whose 

 declination is not more than 60°. 



