Manual" for ships of the Royal Navy, where the conditions are pretty constant, is

1st. Coefficient D. 2nd. Heeling error.

3rd. Coefficients B and C.

But in the Merchant Service, where cargoes change every trip, and where the compensation is often hurriedly made when ready for sea, the "Tentative Method" of adjustment is usually carried out in the following order :

1. Coefficient C (athwartship magnet).

2. Coefficient B. (1) Flinders bar, and (2) fore-and-aft magnet.

3. Coefficient D (soft-iron correctors).

4. Heeling error (vertical magnet).

Coefficient E is rarely compensated, although a counter E could be introduced by moving one of the D correctors forward, and the other a little aft. For +E the starboard corrector should be moved aft, and the port one as much forward; for - E, the reverse.

A table of the remaining deviations is obtained by "swinging" the ship in dock or other convenient place.

Yet, however accurately the compensation may be made, changes in the deviation, from causes already explained, will occur during a voyage, and no careful navigator will place much reliance on "deviation cards." In fact, nothing can relieve him from the obligation to find the deviation from frequent observations, which is now happily made easy by Azimuth Tables.

CHAPTER XXIX.

SYLLABUS.

QUESTIONS ON DEVIATION OF THE COMPASS FOR ORDINARY MASTERS.

THE applicant is to answer correctly at least 12 of such of the following questions as are marked with a cross by the examiner.

The examiner's attention is specially called to the importance of questions 11, 12, 13, 14, and 39, which must be marked in all cases.

1. State briefly the essentials of an efficient compass.

2. State briefly the chief points to be considered when selecting a position for your compass on board ship, and what should be particularly guarded against.

3. What do you mean by deviation of the compass; and how is it caused?

4. Describe how you would determine the deviation of your compass: (1) by reciprocal bearings; (2) by figures on the dock walls; (3) by bearings of a distant object; (4) by the bearings of the sun or other celestial body.

5. 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?

6. Why is it necessary, in order to ascertain the deviations, to bring the ship's head in more than one direction?

7. For accuracy, what is the least number of points to which the ship's head should be brought for constructing a curve or table of deviations?

8. How would you find the deviation when sailing along a well-known coast?

9. Name some suitable objects by which you could readily obtain the deviation of the compass when sailing along the coasts of the Channel you have been accustomed to use.

10. Supposing you have no means of ascertaining the magnetic bearing of a distant object when swinging your ship for deviations, how could you find it approximately from equidistant compass bearings; and at what distance, as a rule, should the object be from the ship?

11. Having taken the following compass bearings of a distant object, find the object's magnetic bearing, and thence the deviation. Magnetic bearing required :—

12. With the deviation as above, construct a curve of deviations on a Napier's diagram, and give the courses you would steer by the standard compass to make the following courses, correct magnetic :

Magnetic courses: S.S.W. W.N.W. N.N.E. E.N.E. S.S.E.
Compass courses required.

13. Supposing you have steered the following courses by the standard compass, find the correct magnetic courses made from the above curve of deviations:

Compass courses: W.S. W. N.N.W. E.N.E. S.S.E.

Magnetic courses required.

14. You have taken the following bearings of two distant objects by your standard compass as above; with the ship's head at W. S., find the bearings, correct magnetic :

Compass bearings: W. by S. and N. W.

Magnetic bearings required.

15. Do you expect the deviation to change? If so, state under what circumstances.

16. How often is it desirable to test the accuracy of your table of deviations?

17. What is meant by variation of the compass? What is it caused by! And where can you find the variation for any given position?

18. The earth being regarded as a magnet, which is usually termed the blue, and which the red magnetic pole?

19. Which end of a magnet (or compass needle) is usually termed the "marked" end, and which the blue?

red or

20. What effect has the pole of one magnet of either name on the poles of another magnet?

21. What is meant by transient induced magnetism?

22. Which is the red and which is the blue pole of a mass of soft vertical iron, by induction, and what effect would the upper and lower ends of it have on the compass needle (a) in the northern hemisphere, (b) in the southern hemisphere, (c) on the magnetic equator?

23. Describe what is usually termed the sub-permanent magnetism of an iron ship, and state when and how it is acquired, and which is the red and which is the blue pole, and why it is called sub-permanent magnetism.

24. Describe the meaning of the expression "coefficient A.❞

25. Describe the meaning of the expression "coefficient B," its signs and effects.

26. Describe the meaning of the expression "coefficient C," its signs and effects.

27. Describe the meaning of the expression "coefficient D," its signs and effects.

28. Describe the meaning of the expression "coefficient E," its signs and effects.

29. Would you expect any change to be caused in the error of your compass by the ship heeling over either from the effect of the wind or the cargo, etc.?

30. The compasses of iron ships being more or less affected by what is termed the heeling error, on what courses is this error usually at its minimum, and on what courses at its maximum ?

The term sub-permanent magnetism in these questions is used in the original sense as proposed by the late Sir G. B. Airy, to denote the character of the permanent magnetism of an iron ship as distinguished from the permanent magnetism of a magnetized steel bar. The terms "sub-permanent" and "permanent" throughout these questions may, therefore, be considered as synonymous.

31. Describe clearly the three principal causes of the heeling error on board ship.

32. State to which side of the ship in the majority of cases is the north point of the compass drawn when the ship heels over in the northern hemisphere. 33. Under what conditions (that is, as regards position of ship whilst building, and the arrangement of iron in the ship) is the north point of the compass needle usually drawn to windward or the high side of the ship in the northern hemisphere; and if not allowed for, what effect has it on the assumed position of the ship when she is steering on northerly, also on southerly courses, in the northern hemisphere ?

34. Under what conditions (as in Question 33) is the north point of the compass needle usually drawn to leeward or the low side of the ship in the northern hemisphere; and, if not allowed for, what effect would it have on the assumed position of the ship, when she is steering on northerly, also on southerly courses, in the northern hemisphere?

35. The effects being as you state, on what courses would you keep away, and on what courses would you keep closer to the wind in the northern hemisphere in order to make good a given compass course (a) when north point of compass is drawn to windward or the high side of ship; and (b) when drawn to leeward or the low side?

36. Does the same rule hold good in both hemispheres with regard to the heeling error?

37. State clearly how that part of the heeling error due to the permanent part of the magnetism of the ship varies as the ship changes her position on the globe; and what is the reason of this.

38. State clearly how that part of the heeling error due to the induction in transverse iron (which was horizontal when ship was upright) and iron vertical to the ship's deck, varies as the ship changes her position on the globe.

39. Your steering compass having a large error, show by "Beall's Compass Deviascope" how you would correct it by compensating_magnets and soft iron (as usually practised by compass adjusters in the Mercantile Marine) in order to reduce the error within manageable limits. Show also how the heeling error can be compensated.

40. As the coefficient B (capable of being corrected) usually consists of two parts, one due to the permanent magnetism of the ship, and the other to vertical induction in soft iron, how should each of the two parts, strictly speaking, be corrected when compensating the compass?

41. If the whole of coefficient B be corrected by a permanent magnet, as is usually done, what is likely to ensue as the ship changes her magnetic latitude?

42. Provided the needles of your compass are not so long and powerful, and so near, as to cause the soft-iron correctors to become magnetized by induction, would the coefficient D, if properly compensated, be likely to remain so in all magnetic latitudes and both hemispheres? If so, state the reason why.

43. State at what distance, as a general rule, the magnets and soft-iron correctors should be placed from the compass needles, and what will be the consequence if they are placed too near the needles.

44. Is it necessary that the magnets used for compensating coefficients B and C should be placed on the deck? If not, state where they may also be placed, and the rules to be observed in placing them into position.

45. Can the compensation of the heeling error be depended upon when the ship changes her latitude? If not, state the reason.

ANSWERS TO THE QUESTIONS ON DEVIATION OF THE COMPASS.

1. The card should be light, accurately centred and divided, and the needles well magnetized, the magnetic axis coinciding with the N. and S. points of the card. The cap should be sound, and the pivot sharp and smooth. The point of the pivot should be level with the gimbals. The bowl should be of copper, and the mounting should not allow of any lateral play.

2. The standard compass should be in the midship line, and should afford a clear view all round for taking bearings, and not less than five feet from iron of any kind, especially vertical iron and any iron which is likely to change its position, such as derricks, cowls, davits, etc. Also as far as possible from a dynamo, if electric lighting is used. Steering compasses should be so placed that the card can be well seen by the steersman, and should be as far removed as possible from the influence of iron.

3. Deviation is the amount of deflection of the N. point of a compass from the correct magnetic N. caused by the magnetic influence of iron in the ship, whether in hull, equipment, or cargo.

4. (1) By reciprocal bearings: An observer is stationed at a compass on shore in a position free from local attraction, and another observer at the standard compass on board. Then, as the ship is swung round, bearings are taken of each compass by the other observer at the same time, whilst the ship's head is steadied on the required points. The difference between the bearing observed on board and the reversed shore bearing is the deviation. (2) By figures on the dock wall: These figures at Liverpool show the true bearings of the Vauxhall Chimney from the river, so that the figure in a line with the chimney as seen from the ship shows its true bearing from the observer's position. The difference between this and the bearing by the standard compass is the total error, from which the variation may be separated, and the result is the deviation. (3) By bearings of a distant object: When the correct magnetic bearing of a distant object is known, the difference between the bearing of it by compass on board, and the correct magnetic is the deviation. (4) By the bearings of a celestial object: The true azimuth of the object being found for a given moment, either by calculation or by tables, the bearing of the object at the same time is taken by compass; the difference shows the error of compass for the actual position of ship's head, and the variation being known, the deviation may be found. 5. If the correct magnetic bearing reads to the right of the compass bearing, the deviation is east; if to the left, it is west.

6. Because the deviation changes with every change of course.

7. Eight equidistant points, viz. N., N.E., E., S.E., S., S.W., W., N.W.

8. By comparing the bearing of two known objects when in one (or in line) with the known correct magnetic direction of one from the other obtained from a chart.

9. (a) English Channel: Bolt Head and Start Lighthouse, Prawle Point and Start Point, etc. (b) Irish Channel: Skerries and South Stack, Smalls and Bishop Lights, Wolf and Longship. (c) North Sea: South Foreland Lights, Whitby Lights, and Tyne Harbour Lights.

10. The ship's head should be steadied on eight equidistant points in succession, and the bearing of a distant object observed when on each point. The mean of these may be taken as the correct magnetic. The object should be seven or eight miles distant, so that its bearing would not be sensibly affected by the swinging of the ship.

11, 12, 13, and 14 are practical.

15. Yes, the deviation may be expected to change: (a) From lapse of time; (b) By keeping the ship's head for a considerable time in an opposite