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DEVIATION OF THE COMPASS.
N.B.—The Candidate is to answer correctly at least eight of such of the following questions as are marked with a cross by the Examiner. The Examiner will not mark less than 12.
The Candidate must draw a curve of deviation on a Napier's diagram, supplied to him by the Examiner. 1. What do you mean by deviation of the compass ?
How much the N of the needle is drawn to the right or left of the correct magnetic N by the influence of the iron in the ship.
2. How do you determine the deviation (a) when in port, and (6) when at sea ?
(a) By swinging the ship, and taking reciprocal bearings with the ship’s head at every point of the compass, by taking the bearing of a distant object whose correct bearing is known. At Liverpool and Cronstadt there are figures on the dock walls for this.
(6) By Amplitudes and Azimuths, and Evans' Variation Chart.
3. Having determined the Deviation with the ship's head on the various points of the compass, how do you know when it is Easterly and when Westerly.
It is Easterly when the correct magnetic bearing is to the right of that by the ship's standard compass, and Westerly when to the left.
4. Why is it necessary, in order to ascertain the deviations, to bring the ship's head in more than one direction ?
Because the deviation is different for every direction of the ship's head.
5. For accuracy, what is the least number of points to which the ship’s head should be bronght ?
Eight : the best are N, NE, E, SE, S, SW, W, and NW.
6. How would you find the deviation when sailing along a well-known coast ?
By choosing two objects which are eight or ten miles distant from the ship, and taking their bearing when in one by the standard compass, with the ship's head on different points. Then the difference between the correct magnetic and standard compass bearings is the deviation for that direction of her head. The Chart will give the correct magnetic bearing.
7, 8, 9, and 10. Note.—These are the deviation curve questions, and must be worked out; they cannot be learned off by heart. (See REED's New GUIDE Book.)
11. Name some suitable objects by which you could readily obtain the deviation of the compass when sailing along the coasts of the English Cbannel ?
South Foreland Lights, Portland Bill Lights, or the Lizard Lights in one ; or any two prominent objects, which are on the Chart.
12. Do you expect the Deviation to change? if so, state under what circumstances.
Yes : for some time after the ship is built ; when she changes her magnetic latitude ; a change in her cargo, particularly if it be at one time iron; keeping on for a long time, and then suddenly changing it; sometimes by a heavy blow from a wave, or collision; and by heeling. 13. How often is it advisable to test the
your Table of Deviations.
As often as possible, particularly in the cases mentioned in the answer to question 12.
14. State briefly what you have to guard against in selecting a position for the compass.
Placing it near any considerable mass of iron, or the extremity of any elongated iron, particularly if vertical, as the capstan, iron stanchions, iron davits, iron funnel, iron masts, &c.
15. The compasses of iron ships are more or less affected by what is termed the heeling error. On what courses does this error vanish, and on what courses is it the greatest.
It vanishes on East and West courses. Greatest on North and South courses.
16. State to which side of the ship, in the majority of cases, is the North point of the Compass drawn in the Northern Hemisphere, and what effect it has on the aasumed position of the ship when she is steering on Northerly, and also on Southerly courses.
The North point is drawn to the weather side. Its effect is to throw the ship to windward of her course on Northerly courses, and to leeward on Southerly courses.
17. The effect being as you state, on what courses would you keep away, and on what courses would you keep closer to the wind in order to make good a given compass course ?
Keep away on Northerly, and keep closer on Southerly
18. Does the same rule hold good in both hemispheres with regard to the heeling error ?
No: in the Southern hemisphere the South end of the needle is drawn to the weather side ; but neither rule is always true, observation is the only way of telling.
19. Your steering compass having a large error, how would you proceed to correct the compass by compensating magnets and soft iron, in order to reduce the error within manageable limits ?
Through the centre of the binnacle draw on the deck two lines at right angles to each other, one fore-and-aft, the other athwart-ships. Put the ship's head N, (cor. mag.) place a magnet with its centre on the fore-and-aft line, and parallel to the athwart-ships line. Move it to or from the compass till the compass shews the ship's head N. Fix the magnet in this position. Put the ship's head E, (cor. mag.) place another magnet with its centre on the athwart-ships line, and parallel to the fore-and-aft line. Move it to or from the compass till the compass shews the ship's head E. Fix this magnet also. The north poles of these magnets should point in the same direction as the North of the needle is drawn. Put the ship's head NE, (cor. mag.) place two boxes, one to port, the other to starboard, of the compass, and on the same level with it. Put sufficient soft iron in these boxes till the compass is again correct.
And shew by diagrams how these magnets and this soft iron are placed.
1st Diagram. Ship's head North or South (Cor. Mag.)
In A, the North point of the compass is drawn towards the starboard side (+C), the compensating magnet's North pole must point also towards the starboard side.
In B, the North point is drawn towards the port side of the compass (-C), the compensating magnet's North pole must point also towards the port side.
2nd Diagram. Ship’s head East or West (Cor. Mag.)
In H the North point of the compass is drawn towards the bows (+B), the compensating magnet's North pole must also point towards the bows.
In K the North point of the compass is drawn towards the stern (-B), the compensating magnet's North pole must also point towards the stern.