1488
THE NEW ZEALAND GAZETTE.
[No. 43

26. What is meant by “the composition of forces” and “the parallelogram of forces”? and show how the knowledge of these is valuable in ascertaining and compensating the sub-permanent magnetism of an iron ship.

27. Describe the nature of the co-efficient B and C plus (+), and minus (—), and the different magnetic forces they represent; also why they are said to produce semicircular deviations.

28. Can semicircular deviations be produced by any other force than the sub-permanent magnetism of the ship? If so, by what?

29. On what points, by compass bearings of the ship’s head, does + B give westerly deviation, and on what points does it give easterly; also on what points does — B give westerly, and on what points easterly?

30. On what points does + C give westerly deviation, and on what points easterly; also on what points does — C give westerly, and on what points easterly, deviation?

31. The value of either co-efficient B or C being given, also the magnetic direction of the ship’s head while she was being built, determine by the traverse tables the approximate value of the other co-efficient C or B; and, the value of both these co-efficients being given, determine approximately the direction by compass of the ship’s head whilst being built, assuming, of course, that these co-efficients resulted altogether from sub-permanent magnetism.

32. Would you expect the greatest disturbance of the needle from the effects of sub-permanent magnetism alone to take place when ship’s head is in same direction as when building, or when her head is at right angles to that direction, and in what direction of the ship’s head would you expect to find the least disturbance?

33. Describe quadrantal deviation, and state what co-efficients represent it; also on what points of the ship’s head, by compass, each of these co-efficients gives the greatest amount of deviation, and why it is called quadrantal deviation.

34. On what points of the compass will each of the co-efficients, D and E, + and —, give easterly, and on what points westerly, deviation?

35. What conditions of the iron of a ship will produce + D, and what — D?

36. State clearly which end of horizontal iron running athwartship (such as beams, &c.), and of horizontal iron running fore and aft of a ship, acquires red and which blue polarity, by induction, when ship’s head is at N.E., S.E., S.W., and N.W. respectively.

37. Describe the nature of the deviation represented by co-efficients + A and — A, and describe the errors in the construction of the compass, and other causes, that frequently produce it.

38. What is the object of compensating the compass by magnets, &c., and what are the general advantages of a compensated compass over an uncompensated one?

39. Before adjusting the compass of an iron ship, what is it desirable to do with the view to eliminating, as far as possible, what may be termed the unstable part of the magnetism of the ship?

40. Describe clearly the tentative method of compass-adjustment (that is, the compensation of co-efficients B, C, and D, with ship upright) as generally practised by compass-adjusters in ships of the mercantile marine.

41. 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.

42. Is it necessary that the magnets used for compensating co-efficients 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.

43. Does the B found on board ship usually arise altogether from sub-permanent magnetism, or does part of it usually arise from some other cause or causes?

44. If the part of B due to induced magnetism in vertical soft iron, as well as the part due to sub-permanent magnetism, are corrected by a magnet alone, as is generally the case, what is frequently the consequence of the ship changing her magnetic latitude and hemisphere?

45. How should each of these two parts of B, strictly speaking, be compensated?

46. Assuming, for the sake of clearness, that your steering compass is unavoidably placed very near to the head of the stern-post (and other vertical iron at the stern), thereby causing a very large — B from induced magnetism, describe briefly any method by which the approximate position for the compensating vertical iron bar (Flinder’s or Rundell’s) could be estimated in order to reduce the error; describe also how you would proceed, in order to improve, if not to perfect, its position after observations have been made on the magnetic equator.

47. State if standard compasses, as well as steering compasses, are generally subject to this disturbance from induced magnetism in vertical iron; also whether the attraction in all cases is found to be towards the stern; and, if not, state the conditions under which it might be toward the bow, and how the compensating soft-iron bar should then be placed.

48. Generally speaking, does the magnetism induced in vertical iron usually have any effect in producing the co-efficient C, ship upright, or is it generally produced by sub-permanent magnetism alone? State also your reasons for saying so.

49. 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 co-efficient D, if properly compensated as you have described (Ans. 40), be likely to remain so in all latitudes and both hemispheres? If so, state the reason why.

50. Under what circumstances does the character of A and E so change as to render it desirable that these co-efficients should be disregarded or modified.

51. Supposing your compasses were allowed to remain uncompensated, explain clearly what would be the probable changes (ship upright) in the deviations produced, separately, by (1) the sub-permanent magnetism of the ship alone, (2) by the induced magnetism in vertical soft iron; (a) on reaching the equator; (b) in the Southern Hemisphere.

52. Assuming you were able to arrive at the proper proportions to be corrected, and were then to exactly compensate the sub-permanent magnetism of the ship by means of a permanent magnet, and the induced magnetism in vertical iron by a soft-iron bar, would you expect any deviation to take place in your compass as the ship changed her latitude and hemisphere? And state your reasons for saying so.

53. Supposing the co-efficient D from horizontal soft iron were allowed to remain uncompensated, would you, or would you not, expect the D to differ in name or amount on the ship changing her magnetic latitude and hemisphere? And state the reason.

54. Describe how you would determine the deviation of your compass—(1) by reciprocal bearings;